Cube 2: Sauerbraten
An Open Source FPS similar in (many) ways to Quake. It's about a 200MB download though, so it may take a while on slower connections. It's got quite a few interesting features though, most notably, a (sorta) easy-to-use, in-game level editor. If configured right, it can run fine on most machines.
If your wondering about the strange name by the way, Cube 2 is the name of the engine, and Sauerbraten is the name of the actual game, not sure why "sauerbraten" though.