Rhino Transport
Look, Rhinos, RRRRRRHHHHIIIIIIINNNNNOOOSSSSS!!!! Our enemies hide in METAL BOXES, DA KOWARDZ! TEH FEWLZ!! We...*Asthma attack* We should take away their METAL BAWKSES!!
....Carron jokes aside, in all seriousness, Rhinos are the primary Armored Personnel Carrier of the Space Marines (both Traitor and Loyalist alike), and are without a doubt one of the most useful and ubiquitous vehicles in the entire Warhammer 40K universe. As an easy bake oven (read STC) produced vehicle, the Rhino dates back to the distant time of Mankind's initial colonization of the galaxy. Originally, they were named "RH1 N0 - Tracked Exploration and Multi-defence. This means that 'L33T SP34K' is still used in the distant grim dark future. Also it being created for "Multi-defence" hints at the possible fact that creation of this STC took place between 1970 and whenever C.S.Goto will finally draw his last breath.
Overview
They are the most ubiquitous vehicle period on the tabletop, as well; they are cheap, effective, easily-modded, and useful in a wide array of scenarios. They give Devastators the ability to get where they need to set up, Rubrics mobility, and if nothing else are something to hide behind when Eldar/Imperial Guard starts trying to snipe your commander with Brightlance/Lascannon fire. They can also be used as battering rams and for Tank Shocking enemy units. Topping it all off, they can pump some ammo down-range for when you really need to squeeze out some extra fire. If you don't take them out while their cargo is inside, you'd better take them before they do such dick moves as acting as moving cover, providing fire support (for the Chaos version,though the loyalists can do the same with the Razorback) and acting as tank shockers, since the annoyance factor of these things racks up in a hurry.
Rhinos aren't terribly well-armed OR well-armored; their front and side armor are AV11, and their rear armor is AV10. Their only weapon is a Storm Bolter (Combi-Bolter for the Chaos version). However, they are impervious to most antipersonnel fire from the front or sides (unless you fight Necrons or Tau), and the fact that they're so frickin' cheap to use - means they see a lot of use in literally every Space Marine and Chaos army that isn't completely drop-pod/teleport happy (and even then they may see quite a few Rhinos). It's so simple that it can be repaired by your average Guardsman (seriously), and its armor is thick enough to hold off considerable amounts of anti-personnel fire - handy against the Orks and Dark Eldar.
Firaeveus Carron hates the things for some reason. Brother Albus of the Thousand Sons has informed us that this was because Carron was dropped on his head during his gene-seeding process. Like, for the entire day he was dropped on his head over and over.
Also, Brother Vold, also of the Thousand Sons, has hinted that the first thing he saw as a mortal was a single Rhino running over a lot of his hometown. Which would be badass if it wasn't so unbelievable.
Rhinos in Action
There are two kinds of Metal Box currently available; the Loyalist Version and the Chaos Version. Both absolutely kick ass.
Rhino Transport The Loyalist Rhino is basic with lots of options. It has a Storm Bolter, which is generally better than a Combi-Bolter at longer ranges, and it can mount a wide variety of weapons as well, including things like Hunter-Killer missiles (infinite range one-shot Krak Missile). Additionally, dozer blades for Loyalist Rhinos work at speeds higher than 6", so it's possible for a Loyalist Rhino to move at cruise speed (or even flat-out) and still get the bonus on Difficult Terrain tests. This makes the Loyalist Rhino useful as an assault transport, and generally ensures that it can get where it needs to go in a hurry.
Chaos Rhino aka The Hedgehog The main difference between loyalist and chaos version of rhinos is that the ones of chaos have SPIEKS! In a few ways, the Chaos Rhino is a better deal than the Loyalist version. It lacks some of the nifty-bang-splat weapons of the Loyalists, but has a Combi-Bolter, which is better at close range than the Storm Bolter of the Loyalist Rhino. It also can pack on more extra stuff to make the model itself larger - which whilst has the potential to make your Rhino a larger (and easier to hit) target, also makes it do its job of providing cover in an emergency that much better. Chaos Rhinos can also mount Dirge Casters (no overwatch for the enemies in 6"), and their vehicle can be possessed (immune to shaken/stunned results but becomes BS3/WS3 in exchange and can occasionally OMNOMNOM unlucky passenger). Its best-known and possibly-coolest advantage over the Loyalist Rhino is the Havoc Launcher, which gives it a long-range twin-linked frag missile launcher that allows the Rhino to join in the fun and spray off some explosives to support its squad. It totally makes up for the fact that Chaos fellows don't get Razorbacks.
Real Rhino The people over at Relic Games commissioned a true-to-scale Rhino for the Grim Dark Present of the Third Millennium. Oddly, they must have only had a Guardsman model on hand for scale purposes (This is because they didn't make one, they dressed up a FV432). See it here. While badass, perhaps the money they put into that Rhino should have been put into Dawn of War II.
Other Rhino variants
Because the Rhino is an incredibly easy vehicle to both make, maintain, and customize, the Imperium has developed several vehicles based on the Rhino chassis, though nearly all of them are exclusive to the Spehs Mahrens.
Predator Tank
The Predator tank is the Space Marine's battle tank that uses the Rhino chassis, but has no more transport capabilities as its weapons take up the space. Instead, it has a turret-mounted autocannon and 2-side mounted Heavy-Bolter sponsons and even heavier armor. The autocannon and Sponsons can be swapped for lascannons for tank-hunting though. There's also the Blood Angels' custom version called the Baal Predator, which mounts a twin-linked assault cannon or a flamestorm cannon for a turret.
Back then, the traditional all-rounder of the Predator was the Twin-linked lascannon turret and H.Bolter side-sponsons. Now, with the changes in the lascannon's stats and price, it's now reversed with the autocannon turret and lascannon sponsons, as this is now the cheaper option. If you use the 4th edition combo now, you're probably better off with a Land Raider.
Razorback Transport
A heavier Rhino that eschews some personnel-carrying capacity for heavier weaponry. More expensive, but it can now mount shinier bits like Multi-Meltas and Lascannons, which means it can act as a useful close-support unit as well as a transport, if the Predator is your Main Battle tank, the Razorback is your light Infantry Fighting Vehicle.
Whirlwind
The Whirlwind artillery tank is the Spehss Mahrens' primary artillery piece. It can't exactly contest with the IG's artillery in terms of power, but it is more mobile than most of them and can fire 2 shots per shooting phase (why, exactly? It's got two missile types, but not two shots...). It's optimized for screwing over infantry, it can either fire an AP4 shell for anyone in the open, or another one that ignores cover saves.
Vindicator
Probably the only good thing Roboute Guilliman did during the Horus Heresy is inventing the Vindicator. The Vindicator is the nightmare of every other thing out there. The Vindicator mounts a Demolisher Cannon, which is basically a S10 AP2 pieplate shotgun and coupled mad-tough frontal armor.....with a fucking dozer blade that makes it look cool and makes sure that even difficult terrain can't stop it from fucking someone's day up. In short, think of it as a battering ram ... With a Demolisher cannon. If something expensive gets in it's firing range, it's 98% guaranteed dead, or at least messed up badly. With 6th ed rules any vehicle even touched by the deathpie template gets the full brunt of fuck, instead of just half.
It's even scarier when it's with Chaos version, throw in a havoc launcher and daemonic possession and then watch your opponent devote retarded amounts of firepower to keep the Khorne-incarnate rape-engine down.
Stalker
A new Pattern, introducted in the 6th edition codex for the Space Marines. Looks like an overhauled Whirlwind, but replacing the Missile launcher with Anti-air cannons and MOAR plates, making it a Flak tank of sorts. New info in september with the release.
Sabre
A fairly old Pattern, introducted in the Rogue Trader days for the Imperium. This tank has a Vanquisher cannon or a laser destroyer and is pretty good at fucking up armoured columns. Think of a Jadgpanther and your pretty close.
Immolator
A Sisters of Battle only variant, the Immolator mounts a pair of Heavy Flamers standard, but can switch it for a pair of Heavy Bolters or a pair of Multi-Meltas, thereby completing the Holy Trinity of Bolter/Flamer/Melta. Formerly a go-to unit for anyone playing Witch Hunters, but when the Inquisition got split off and Sisters got their own separate army, the points got jacked up and it stopped being as popular. It came back in force with the 6e digital update, with a points decrease, every unit in the army having easy access to it and a free upgrade to multi-melta.
It's worth pointing out that it, like the Sister Rhino variant you can build from the same box (protip: The difference is whether the glass canopy on top is open or closed) are absolutely gorgeous models. But then, having gorgeous models has never been an issue for Sisters.
Deimos-Pattern Rhino
An earlier version of the Rhino used during the Great Crusade, the Deimos-Pattern Rhino is somewhat better armed than the currently used version with two turret-mounted bolters (which could be replaced with a pintle-mounted combi-bolter, heavy bolter, heavy flamer, or Havoc missile launcher). However, this extra firepower comes at the expense of front armor, and without an assault ramp its not quite as good at carrying assault troops into battle.
Rhino Advancer
The Rhino Advancer was a long bodied, open-topped variant of the Deimos-Pattern Rhino modified to carry a greater number of Space Marines while also letting them deploy more rapidly. This was due to the larger, 20 man Tactical Squads of the Crusade/Heresy Era. They have since fallen out of use as their increased capacity was of little benefit after the Legions were divided into smaller Chapters.
Grav-Rhino
This Rhino was specially equipped with grav-plates similar to the Land Speeder and further enhanced with technology reverse-engineered from Eldar transports, allowing them to hover over the ground and fly to a limited extent. Only the Adeptus Custodes and the Sisters of Silence have been known to use them, and even then they have never been deployed since the Siege of Terra. It's been theorized that they were meant to carry troops to and from the Imperial Webway that the Emperor was attempting to construct.
Castellan Rhino
Like anything else associated with the Iron Warriors, the Castellan Rhinos were designed with siege warfare in mind. While it was still perfectly usable as a transport, its real value stemmed from the unfolding armor bracing that allowed it to be quickly repurposed into an instant bunker.