Cube, the Free online FPS. This is pretty much a free version of Quake. Assault Cube is a mod of Cube, and is pretty much a ripoff version of Counter Strike. If you're going to be playing normal Cube, play Cube 2, because there's hardly any activity on Cube 1.
The neat thing about Cube is that it's engine is designed to be both a map editor and a game engine. When in an editing room, or in the default lobby, press E to start editing the map. There are rooms up on the server list that are for the sole purpose of having people edit the map together. I don't beleive the editing function is possible in Assault Cube, though.
Cube 1 Cube 2: Sauerbraten Assault Cube
-----
Cube 2 map Download pack Official Website
Very neat free FPS(s) 
x2
Linear Mode
