Geez, I have too many to count. I'd say
Urban Terror. It's 1 glorious gigabyte of space but it has a ton of maps and weapons, knife throwing, as well as maybe dual-wielding in a future update. If you don't want that many weapons or maps, or not enough space, choose
Assault Cube, based off of
Cube 2: Sauerbraten (free and has ragdoll physics, though rather outlandish). Another, sci-fi, Cube 2-based game is
Red Eclipse, free and has ragdoll physics, as well as modes for extreme customizability (adding jet packs, melee-only, choosing weapons at the start, teamplay, etc...). If none of those take your fancy,
Tribes and
Tribes 2 are free, as well as the upcoming
Tribes: Ascend.
NOTE FOR TRIBES: Vivendi shut down the official server. Tribes does not run multiplayer and Tribes 2 has a patch called "TribesNext" that you can use to play online. Tribes 2 links to the TribesNext website. Download and install the bottom download, then the top download IN THAT ORDER.