Sauerbraten is multiplayer/singleplayer FPS game free game project, and soon an RPG game project. The sourcecode for the engine used in these games is Open Source.

Game features:
* 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

Engine features:
* 6 directional heightfield in octree world structure allowing for instant easy in-game geometry editing (even in multiplayer, coop edit)
* rendering engine optimized for high geometry throughput, supporting hardware occlusion culling
* lightmap based lighting with accurate shadows from everything including mapmodels, smooth lighting for faceted geometry, and fast compiles
* pixel and vertex shader support, each model and world texture can have its own shader assigned
* robust physics written specifically for this world structure
* loading of md2/md3 models for animated characters, weapons, items, world objects.
* Network library designed for high speed games, client/server network system
* small but complete configuration/scripting language
* simple stereo positional sound system
* particle engine, supporting text particles
* Simplistic menu system, console, etc.