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)
Gears of War
Tom Clancy's Splinter Cell
Tom Clancy's Rainbow Six
Brothers in Arms