Dread Necromancer
A Dread Necromancer is a necromancer who's more of a necromancer than other necromancers. In fact, other necromancers wish they were as much of a necromancer as a dread necromancer. They also sometimes wear dreads. I dread to think that this is all anyone could come up with for this class... *BLAM!* Levity is heresy in optimization!
Seriously, kids, Dread Necromancer is considered by and far the actual way you create an optimized necromancer for 3.5. It basically gets nothing but good shit for class abilities/spells, and there's a million ways to break it in half. This class and Archivist are pretty much the best parts of Heroes of Horror; while Archivist leads to some truly broken shit (as in some Tier 0 table-flipping bullshit that doesn't even take a lot of work to figure out), Dread Necromancer is basically more "civilized" because it won't really break a game so much as make sure that you get to play the kind of necromancer that dozens of supplements from 2nd edition to 2.5 have tried - and largely failed - to deliver on.
Basics
On the surface, a dread necro is an Cha-based arcane caster with shitty BAB, good Will saves, hit points like a bard, and poor amounts of skill points. They get light armor and can cast in it, and they get all simple weapons plus a martial weapon of choice (greatsword if you're going full optimization, flail or scimitar for style+optimization, and scythe if you want to look cool but be absolute shit in combat). They look a lot like a kind of sorcerer, but there's two differences that make them superior.
The first difference is that a dread necro gets a Goddamn fuckhuge list of class abilities. As in, they get some shit every fucking level. This is one of the few classes in all of 3.5 that doesn't have empty levels, and it's not even like the abilities they get are shit. No, dread necro gets good stuff every level. Okay, Fear Aura isn't stellar, but it's better than nothing, and you even get it at a level that makes it pretty effective (i.e. chases off little mooks, leaving BBEGs ripe for the taking).
I mean, Jesus, look at the 1st-level ability Charnel Touch. Christ, it's negative energy touch damage on command. Sure, you're BAB kind of sucks... but touch attacks bypass about %50 of the shit you'd have to worry with on AC, and there's delicious ways to boost your attack roll (which we'll cover later). Oh, wait, negative energy also heals all undead. And your class is meant to make undead minions to aid you in combat. So... unlimited healing of undead? Yes, please.
But it just gets better. You get Rebuke Undead, which is fucking stupid for you. Not only can you just steal other necros' undead from them and they can't do much of shit about it (unless they also have Rebuke), but you now get access to Divine Metamagic bullshit that is usually reserved for CoDzillas. Never mind that Liber Mortis and a few other tasty supplements give you ways to bolster your undead even more with your Rebuke.
This is all at 1st level, guys. Level fucking 1.
After that, it just keeps getting better. You slowly start to turn into a lich yourself, gaining DR and fortification (i.e. protection from crits), until you cap off as an actual, factual lich at 20. (Note that this isn't always optimal, but we'll cover that later.) You also start getting enhancements to your Charnel Touch, adding disease and eventually negative levels (not permanent, but still). Negative energy burst to hurt the living and heal your undead; advanced learning to add any cleric or wizard necromancy spell you don't already have to your own list; a familiar which is automatically an improved version over the usual shit (and can deliver your Charnel Touch for you); you even get to control more undead based on your Cha bonus, so you can have more (bigger) minions.
Second to your class abilities is your spell list. You don't get any 0-level stuff (boo), but you get as many spontaneous spell slots as a sorcerer (yay). You don't get to pick your spells (boo), but you automatically know EVERY SPELL of a given level you can cast (lolwut?), and they are a mix of some of the more ridiculous necromancy spells from both arcane and divine sources. You also get access to a few new things from Heroes of Horror like the summon undead variants, allowing you to temporarily shore up your more permanent minions with some cheap, disposable mooks. Oh, also, you get several good touch spells, and you can use spectral hand to deliver Charnel Touch at range.
And just like that, you are a Goddamn supervillain.
Advanced Builds
Honestly... there's not a hell of a lot to do here. If you dip out, you miss out on bonus Charnel Touch damage/healing and Rebuke per-days. So you need to be very clear about why you DON'T want to go all the way with this bad bitch.
One reason not to stick it out is pretty compelling: lich isn't necessarily the "best" undead package for you. Why? Because as early as level 3 or 4, you can become a necropolitan (LM pg. 114) which enjoys a lot of tasty undead benefits (along with great bonuses like automatic slow healing per-day like the living, and Turn Resistance which is clutch to avoid problems with do-gooding clerics), and doesn't affect your Level Adjustment in the slightest. If you do this early enough, you basically just spend a few thousand XP to become undead and then it's full XP ahead: you'll be a half-step behind the rest of the party. In fact, necropolitan isn't even expressly Evil: they crucify themselves to become undead, and do not have any unnatural hungers for blood, flesh, level-drain, etc. Unless the DM makes an issue of it, being a necropolitan is basically the most party-friendly kind of undead you can become.
Note you can't do both things. If you go to dread necro 20, you become a lich due to the mechanics. You overwrite your necropolitan package, which means no self-healing over time. Fuuuuu...
Lich isn't "bad", don't get it wrong. They get a slight bonus to all mental stats and some great defenses along with nice offenses. But at +4 LA, it's just not worth comparing to getting four more levels of... well, anything, kind of. Heck, if you know you're going to at least 20 and maybe a few levels beyond, you can easily find some classes that have Cha synergy and/or boost your abilities.
So yeah, keep your status as undead in mind as we talk about some options for your career.
The first and most obvious choice of prestige class is Pale Master. In fact, it's a pretty good fucking choice of career for a dread necro: its abilities dovetail nicely with the dread necro. The first level is shit: no spellcasting progression, no bonus anything, just a couple of paltry hit points and skill points. But after that, it's golden with class abilities and spellcasting every level. In fact, the first thing you get is per-day animate dead as an SLA, meaning you can create permanent minions without cost. Darkvision and deathless vigor are swell, giving you better perceptions as well as a Fort save bonus that will almost always apply to stuff; your control undead ability, in contrast, is weak and silly. Undead armor affinity is conditionally good: if the DM lets you further customize the Undead Armor items listed in the book with additional item enhancements, then do so and enjoy, otherwise just stick to mithral medium armor and customize it. But undead graft and deathless mastery are really great. The graft gives you an additional source of touch attacks, and at 10th level this includes the ability to kill someone and turn them into a zombie under your control... beyond your usual limits on undead minions. So you could, with patience and time, just animate the shit out of hundreds of minions, bury the fuckers, and wait until you need to whip out an army of zombies, backed up by your other-other undead minions (the really good ones with a HD cap), plus other shit as you come up with it. Best part here is, you can either go undead or not, because your pale master 10 ability makes you quasi-undead with loads of immunities but you can technically still be alive. Note that you WON'T get Rebuke progression with this, but with unlimited no-cost animate dead and literally endless zombies, it's a moot point; use Rebuke for DMM shenanigans or Profane stuff.
Another very cool and interesting option is Fiend-Blooded, also from Heroes of Horror. It's not as "pure" necromancy as the Pale Master, but it's honestly not bad at all. The capstone at 10 is a little worth the drop in spellcasting, since you'll still get to 9th-level spells, and it gives you a nice little mix of abilities. Of special note, you get a better form of advanced learning, letting you put fire-subtype, enchantment, illusion, or necromancy spells in your list; this allows you to get some neat things like blast of flame (a conjuration spell from Spell Compendium that allows no SR), or some good illusions like (greater) mirror image.
Now. This is all if you're strict dread necro. But what if you want to mix things up with even more classes?
Dread necro, warlock, and eldritch theurge are amazing together. The synergy with spellblast and eldritch spellweave allow you to really do some bullshit attacks at range, where the choice is "die quickly" or "die MORE quickly". The basic choices are dropping area effects on eldtrich blast targets, or eldritch essences to targeted spells. But the real trick here is that you advance your eldritch blast and your spellcasting simultaneously. Granted, you don't get all the fun of dread necro abilities, but invocations more than make up for that, especially when you get stuff like baleful utterance, fell flight, flee the scene, and chilling tentacles.
There's also another good option here that could be worth it: ur-priest and true necromancer. Ur-priest gives you rebuking and full cleric-style spellcasting, both of which are killer for a dread necro. True necromancer allows you to advance both arcane and divine spellcasting, and You do it by going dread necro 8 / ur-priest 2 / true necromancer 10. You do lose on your necro abilities, but what you get is two sources of 9th-level spells, and with ur-priest you get to select whatever spells you actually want. Best part is, you don't actually gimp your dread necro spellcasting that bad; you loose only 2 levels of progression, but full rebuke ability. Some of the true necro abilities are also pretty good to mix in with dread necro by that point: you get bonus caster level for rebuking and necromancy spells/SLAs, and a radius desecrate effect (which makes it harder to turn undead, and gives undead bonuses to attack, damage, saves, and hit points). You even get a per-day create (greater) undead at various levels, so you don't have to blow a spell to make yourself a good minion.
Good dip PrCs are Fatespinner, Mindbender (if you can get a charm spell into your list, don't forget to get Mindsight later), Abjurant Champion (it's not strictly "optimal" because it doesn't do shit for necromancy, but some guys want to gish everything, so...) and/or Spellsword.
Time to talk feats now. Arcane Strike is, in fact, a good choice. Not only will it help you land your touch attacks easier, but they'll hit even harder. Consider getting both Arcane Strike and Improved Unarmed Strike, possibly even Superior Unarmed Strike (Tome of Battle) so you can be some kind of fucked-up "Necro Fu" guy. You can even do this as an eldrtich theurge or a true necromancer: burn your arcane slots for bonuses, use your invocations or divine spells for buffs and other stuff.
Of course, getting into melee could be considered a bad idea, and for good reason: dread necros aren't exactly the beefiest people. Plus, let's not forget, the point is to create minions to do the bulk of your fighting for you. So, consider going with Divine Metamagic, and stuff like Quicken Spell, Silent/Still Spell, or even Heighten Spell (which makes the spells "effective" level higher; for example, you make a 2nd level spell count as if it were a 3rd or higher level spell). Heighten is neat because it makes it harder to resist the spell, and it can also overcome a lot of special protections that stop spells of a certain level or lower. Reapeat and Twin spell are also great choices there. Get Sanctum Spell for your big ritual stuff, like creating undead or even planar binding spells.
Another great choice are the feats in Liber Mortis. Corpsecrafter and the subsequent feats under it can be great: bonus Str, hp, turn resistance, natural armor, even initiative and movement bonuses, all are great for hordes of minions meant to whittle away at enemies. Necromantic Presence/Might are also great, giving bonuses to any undead you have control of, even if you didn't create them. If you don't want to become undead (but why wouldn't you?), consider taking Tomb-Tainted Soul and the other spells under it. If your DM is stupid enough to let you get Undead Leadership, cackle maniacally as you win the entire game; the cohort and followers do not count against created undead you make. Profane Vigor is a useful burst-heal for your minions around you. By far, Fell Drain is the best feat in this book, since negative levels are a quick way to solidly fuck most opponents over. Fell Animate is a fun one: use it to wipe out mooks and then turn them into minions under your control. Enervate Spell is pretty good, too, but since it costs the same as Fell Drain (which is more than arguably better in most ways), it's considered an inferior feat. If you decided to become undead early, take Improved Turn Resistance just in case, and Positive Energy Resistance to avoid the unpleasant encounter with a goody-two-shoes cleric. Lifesense is interesting to take. Lifebond is only useful with a living ally, but taking it with a fellow PC can lead some very interesting role-playing.
Draconic Heritage stuff is technically feasible, but not as optimal. Most of your dread necro spells are pretty fucking good; only constructs are your real bane. (Why not undead? Because, you silly boy, you can use rebuke/command against them! If you like someone's minion, may as well take it from them before they die...) With constructs, you let your fellow PCs help you out, or if you're just a complete emo who can't depend on others for help, send in your properly-armed undead minions (that's what they're there for, after all) or some planar beings you bound to service (also something you can do, similar to a wizard). Still, the draconic stuff isn't bad by a long shot, but overall not "as good" as other things.
Items are pretty self-explanatory: anything that helps your creation, control, and buffing of undead. But here's the funny thing: you're not really here to buy items only for yourself. Instead, look to spend some of your wealth on good gear for your best minions. Armor, weapons, and other stuff can turn you boys from road-kill for the BBEG into the actual things bringing them down.
The best undead to make are bone or corpse creatures (Book of Vile Darkness, uses create (greater) undead); dread warriors (Unapproachable East, requires you go to add animate dread warrior to your spell list); nightmares (because they get to keep their awesome flight ability even when animated as mindless skeletons); zombie dragons (they keep a weakened version of their breath weapon); giants (as combat mooks). Don't underestimate having a few weaker minions to do chores and stuff; kobold skeletons are delightfully spooky and cute!
Remember, your biggest strength is what you create and/or summon, not just your own powers. Use your abilities to enhance and boost your minions, and proof them against various kinds of destruction, and you'll go far. If you loose minions, "live off the enemy" and create new minions from your defeated foes.
Being a dread necro means never running out of friends willing to help you out...