If you have played Doom, Quake, Unreal Tournament, or any of their various free mods/clones, then you already have a good idea of what to expect from Nexuiz. Like all those games before it, Nexuiz is simple, dumb fun.
Hedgewars is multiplatform, a free turn-based blast-em-up in the vein of Worms. It features a wide range of weapons, multiple maps, and excellent graphics and audio. Offers single player, multiplayer, and (in theory) multiplayer over the internet modes.