The codename for post-Unreal Tournament builds of the Unreal game engine (to be specific, versions after build 613). It is still under development, although most games using it have now forked stable versions.
It is available on Windows, Linux (currently only as a dedicated server), Playstation 2 (ported and maintained by Secret Level) and XBox platforms.
Some of the innovative new features include:
This version of the engine is being used in Deus Ex 2, Unreal Tournament 2003 and Unreal 2, as well as many other games.