Sauerbraten (a.k.a. Cube 2) is a free multiplayer/singleplayer first person shooter, built as a major redesign of the Cube FPS.
Much like the original Cube, the aim of this game is not necessarily to produce the most features & eyecandy possible, but rather to allow map/geometry editing to be done dynamically in-game, to create fun gameplay and an elegant engine.
Oldskool fast & intense gameplay (read: similar to Doom 2 / Quake 1).
Many multiplayer gameplay modes, most in teamplay variants as well: deathmatch, instagib, efficiency, tactics, capture (domination/battlefield style), coop edit (!).
Masterserver & ingame server browser.
Lag-free gameplay experience.
Two singleplayer modes: DMSP (fight a monster invasion on any DM map), classic SP (progression driven SP like other games)
7 weapons tuned for maximum satisfaction: double barrelled shogun, rocket launcher, machine gun, rifle, grenade launcher, pistol, fist.