Obliterators
These big motherfuckers are Chaos Space Marines who have been infected with the so-called "Obliterator Virus" (Oh so subtle GW...Oh so subtle), which basically turns them into a giant ass walking Daemonic Engine of Mass Rape. It's a large walking blob of Termie armour, daemonically enhanced killing machine, and guns ...BIG guns.
Obliterators are rare (usually) and very sought after, due to their complete ability to rape everyone and every thing within a 5 mile radius. They don't work for any particular legion or worship any Chaos God individually, but they have their own insider cult with the pursuit of knowledge, alchemy and the fusion of flesh and Stehl. NOT TODAY, ZURG. Usually having a connection to techmarines or techpriests they get some KICKASS powers along with the virus, however it also turns them batshit insane. Any object they hold can get merged into them, allowing them to copy them AND produce ammunition for them....Tyranid Digestion Pool?! (Which is kind of fucking creepy if you didn't know you were infected)
The metal they get infused with turns them durable as fuck, (Wolverine anyone?) although very slow, but they have teleporters so it's all good. The Iron Warriors have a shit ton of these guys just waiting for commands due to some connection with the Obliterator cult.
Overview
Obliterators are probably the most used heavy support choice in the Chaos Space Marines 4th and 5th edition Codex, and with good reason. During each shooting phase, they have a choice of using a twin-linked Flamer, a Heavy Flamer a twin-linked Plasma Gun, a Plasma Cannon, a twin-linked Meltagun, a Multi-Melta, an Assault Cannon or a Lascannon, meaning it matters not what the enemy is, they have a means of dealing with it. They also have Powerfists and a 2+ armor save (plus a 5+ Invul) and 2 wounds, though since they can only be fielded in groups of three at the most, it's best to keep them out of melee, since you want them free to shoot at enemies.
Their drawbacks are that aside from an extra wound, their statline is the same as a regular Chaos Marine's, meaning they can be hit with Instant Death from a number of heavy weapons. Like Terminators Thousand Sons, they also have Slow and Purposeful, so it can be a problem getting them where you need them. Also, all Obliterators in a unit have to fire the same weapon, no mixing and matching, so make sure you know what weapon you want to use.
The 6th edition book, while trying adjust units to avoid the problems where the 4th edition had some units that were so good it was pointless to use anything, actually gave some improvements to the Obliterators. They can now be given Marks of Chaos, and they can finally use Assault Cannons (though rending has taken a hit). The only drawback they gained was that they can't use the same weapon two turns in a row, which isn't all that big a deal, especially since they have some overlapping weapons (Use a plasma cannon one turn, and then TL plasma guns to clean up the next). Though improvements to other units does mean that now they do actually have some competition in the shooting area.
Oblits were outrageously more dangerous in the 3.5 edition Codex due to their S/T5! However, their toughness got FAQ'd down to 4(5) (meaning at the time no immunity to instant death S8) and although they didn't have a plasma cannon or multi-melta, they had access to heavy bolters and frag missiles. Obliterators gained infamy through the notoriously broken Iron Warriors list that spammed them and used Basilisks to decimate anything thrown at them before it could even shoot back. It's no exaggeration that these guys were among the reasons that the 3rd edition codex was considered to be so overpowered that GW had to release 4th ed codex to balance it.