Splash Damage - Wolfenstein Enemy Territory
ET Ultimate Installer with updates, modes and maps.
The source code for the game is available at ID software.
The people behind ioquake3 took up the job of modifying the code, see Welcome Wolfenstein: Enemy Territory and Return to Castle Wolfenstein!, but there hasn't been much progress: icculus.org - BugZilla - iowolfet.
ET: Legacy, Wolfenstein Enemy Territory: Legacy Project with src on github.
ETXreaL is a graphics mod for Wolfenstein: Enemy Territory using the enhanced XreaL id Tech 3 GPL engine.