Fel Reaver concept art
Fel Reaver in-game
The Fel Reavers are enormous demonic machines found in Outland. They are immensely powerful foes and are constantly emitting fel green smoke.
Fel Reavers cover ground with incredible speed, due to their long legs and an internal power core that runs on fel fire (as opposed to nether gas). While questing in Hellfire Peninsula, it's quite common to have one of these giant engines of destruction sneak up behind you. If your screen starts shaking and you hear a noise somewhere between a steam engine and a roaring beast, find shelter! These towering behemoths, luckily, have a small aggro radius for a Lv. 70 elite.
Currently not much is known about the Reavers' construction or origin. The Burning Legion use them as sentinels all around Hellfire Peninsula, including around their forges to the north to oversee the work on their cannons.
Reaver's Fall, in eastern Hellfire Peninsula, is the location of remains of a destroyed Fel Reaver, with a handful of trolls, orcs, and tauren cheering and dancing around the fel debris. At this area, players can execute bombing runs against the legion front.
Another area, Expedition Point, is near the gateways of The Legion. It's also built around Fel Reaver parts that are being forever hacked away at to bring to Stormwind to be melted down and remade as weapons to use against the Legion.
There are fel reavers parked at Forge Camp: Terror and Forge Camp: Wrath in the Blade's Edge Mountains.
Tactics
A Fel Reaver can be downed by a three man level 70 party composed of a tank, a healer, and a DPSer. While it does take a while, the damage output of a Fel Reaver is comparable to that of any random 70 elite and completely focused on the tank (in other words, surprisingly low for something of this size). As of version 2.1.0, stealthed rogues and druids within the ground-shaking range of the Reaver, even if outside aggro range, lose stealth, seemingly with each shake/roar.
To obtain the Key for Shattered Halls, you must kill a Fel Reaver and forge the key in its fiery carcass.
A Fel Reaver may also be pulled while just northeast of Thrallmar. If kited to the wrecked caravan just outside of Thrallmar, after which the player drops aggro through a skill such as 
[Ice Block] or 
[Feign Death], the guards will pick up aggro on the Fel Reaver, and eventually kill it, allowing you to solo this quest with relative ease.
The Bear Reaver
Fel Bear Reaver ... thing?
In the patch applied to the beta server on the 3rd of November 2006 the model used by the Fel Reaver which patrols Hellfire Peninsula (others seem unaffected) was replaced with a black bear. The hilarity which has ensued is epic in proportions. The screen still rumbles as it approaches, and it still sounds like a giant train. It also retains the power to annihilate anyone in 1-2 swings, but ... it just looks so cuddly now! Bear puns are rampant and Hellfire Peninsula's general channel verges on total collapse!
Even before the change the Reaver was a menace to unwary players, but this has added a new level of terror to an already overwhelming foe. Formerly, the Fel Reaver, being a good five stories tall, was visible from a fair distance away and easily avoidable to anyone aware of the danger. Now, however, the first warning of its presence is the terrible rumbling of its footsteps and the tortured metallic scream which precedes it. Many heroes have fallen to this new menace, frozen in panic as they try to determine which way to flee. Truly, our greatest enemy is upon us.
Famous Fel Reavers
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