Urban Terror is a total conversion for Quake 3. And a realy nice one! But you still have to own Quake 3.
Not long anymore...
Silicon Ice (developers of Urban Terror) has announced to port Urban Terror from Quake 3 to Return To Castle Wolfenstein: Enemy Territory, Activision's free multiplayer version of
RTCW (click on Enemy Territory). Development was halted on RTCW: ET because time and money, but what was finished was released for free (a fun MP game!)
A couple weeks ago, Activision released their
source-code for RTCW: ET giving modders al over the world a chance to make mods for a free high-end multiplayer platform.
This looks like fun!!