A really powerful game engine used by Epic Games. Very beautiful and popular engine, one of the very best engines out there for video games. It very popular because it supports practically any platform it's put on (PC, XBox, Gamecube, PS2, etc... and even Mac)

A great deal of the gameplay code is written in UnrealScript, a proprietary scripting language, and as such large parts of the gameplay can be modified without delving deep into the engine internals. As with other middleware packages, the Unreal Engine provides various tools to assist with content creation for designers and artists.

It has 3 versions: 1 (1998-2002), 2 (03-06), and 3 (06-future)
Some games/franchises that use Unreal Engine

Unreal Tournament
Unreal Championship
Gears of War
Red Steel
Deux Ex
Tom Clancy's Splinter Cell
Tom Clancy's Rainbow Six
Mass Effect
Lost Odessey
Too Human
Brothers in Arms