XMP was originally a total conversion for Unreal 2 and was created by Legend Entertainment who have since closed.

http://www.free-monkey.com have now taken on this project for UT 2004 with support from some of the old Legend Entertainment team and are offering the first beta of the Expanded Multiplayer for UT 2004.

Unreal Tournament: Expanded Multiplayer will offer intense class-based vehicular action in a range of stunning alien worlds, where players must manage energy reserves to power their war machines and battle for the control of four mysterious artifacts. UT XMP players will be given control over a vast array of all-new fearsome weapons, as well as mines, turrets and other deadly automated defenses.

A list of mirrors for download are available here.

Just to point out that Free Monkey are a non-profit team of Unreal enthuiasts and a number of its members have been producing Unreal content for some time, so hopefully if you have UT 2004 this one might be worth a look.