Beguiler
The Beguiler is a character class for 3.5 Edition D&D printed in the 'Players Handbook II.
Its a sort of hybrid caster/rogue in the same way that the Spellthief class is. But just as the Duskblade is to Hexblade (and indicating a trend between PHBII and the Complete... series of books) the Beguiler has much more broad application for a party and fits in more smoothly.
The class has a "full" spontaneous INT-based spellcasting progression up to 9th level spells, however they are only chosen from a restricted spell list which basically only includes Enchantment, Illusion and Transmutations spells with a few other useful Abjuration spells thrown in for good measure. So the spell list is basically a toolbox that covers common situations and are basically the inverse of the Warmage character class, so where one can ONLY do damage, the other one finds it very difficult to do so.
To assist the Beguiler in spellcasting they may cast in Light Armor unhindered, and gain Silent & Still Metamagic feats as part of their class progression, all the better to aid them in casting spells sneakily.
They can also feint in combat and catch opponents flat footed against their spellcasting, which also coincides with another class feature "Cloaked Casting" which enhances their spellcasting against such opponents, just remember that very few Beguiler spells cause damage.
Finally they also get Trapfinding and a decent list of skills and skill points, meaning they can provide some usefulness to the group outside of spellcasting.
The downsides to all this are a terrible BAB progression, and the only saves are Will based, but that's pretty much what you get when you have a 9-level spell progression so its not all that bad.
Making Beguilers work
The Beguiler is a fairly inoffensive class, while it doesn't have the raw power of a wizard/cleric it does make for a decent character. While outside of combat they do very well as the "face" of the group, and perhaps even perform better than Bards so with their large selection of spells and skills.
Their best use in combat is trying to manipulate the battlefields and turning opponents against themselves, so gaining feats and abilities which improve their ability to do this is especially useful.
Its a class that doesn't lose too much by multi-classing out, since the "Cloaked Casting" ability isn't entirely awe-inspiring.
One prestige class that stands out above all others is Rainbow Servant from Complete Divine, as just like the Warmage combination, this makes the Beguiler utterly replace divine spellcasters by the time they complete the class, since their spell list is entirely fixed (unlike a Sorcerer) and they cast spontaneously. The addition of Divine domains also grants them more versatility spells in while they wait, which is a further win.
Other prestige class options are viable though.
For players looking to a bit more of a martial bent, the Abjurant Champion prestige class from Complete Mage can be qualified about halfway through their career since they have some martial weapon training (Rapiers) and Undetectable Alignment is a lv1 Abjuration spell, the only sticky prerequisite is a BAB of +5. This massively enhances their BAB progression when they finish the prestige class and allows them to boost their damage output by burning unused spell slots.
In a similar vein, Beguilers also easily qualify for the Unseen Seer prestige class from the same book, since they have just enough Divination spells in their list to grant them access. This makes them much more Rogue-like by giving them access to Sneak Attack, a slightly improved BAB, permanent Nondetection and expands their spell list to include freely selectable Divination spells.
In fact, for your consideration: a combination of Beguiler 5/Seer 10/Champion 5 is mathematically awesome, since they have a full +20 BAB, 9th level spell progression of mostly illusion spells that can buff/debuff (and so can do well in combat), the ability to burn spell slots for more damage, sneak attack and a good amount of skills. Not quite as epic as the Rainbow Servant, but still a great alternative combination.