Sauerbraten is multiplayer/singleplayer FPS game freeware game project, and soon an RPG game project. The sourcecode for the engine used in these games is Open Source (ZLIB licence, read the "License" section below carefully before starting ANY kind of project based on this engine).
You will want to read (roughly this order):
G a m e - D e s i g n :
P r o g r a m m i n g :
Additional code by : Adrian "driAn" Henke (md3 code), Jerry Siebe (geometry rendering optimisations), Julian Mayer (MacOSX port)
L e v e l - D e s i g n :
Additional maps by : Aardappel, driAn, Gilt, voot, shadow, staffy, JCDPC, ZappaZ, MakkE
A r t :
Additional art by: metlslime (textures), Than (textures), Remedy Entertainment Ltd (textures), Seth & Ulrich Galbraith (GPL models), Brian "EvilBastard" Collins / Conrad / Magarnigal / Psionic / James Green / Andreas Möller / Ryan Butts & Jeramy Cooke (md2 models)
S o u n d / M u s i c :
O t h e r :
The Sauerbraten game is freeware, you may freely distribute the Sauerbraten archive/installer unmodified on any media. You may re-compress using different archival formats suitable for your OS (i.e. zip/tgz/rpm/deb/dmg), any changes beyond that require my explicit permission.
You may play Sauerbraten for any purpose as long as you don't blame me for any damages incurred.
You may produce new content for sauerbraten either as a "custom map" (.ogz/.cfg/textures etc) or as a "mod" (same, but with new exe), but you can only distribute this content as an add-on archive that contains NO existing sauerbraten content and installs into an existing sauerbraten installation without overwriting files. You explicitly may NOT repackage the game's media with your own content, only the engine is Open Source, the media that comes with the game comes with varying licenses from varying authors. If you wish to build your own game based on the sauerbraten engine, you therefore have to make it work entirely with either your own content, or content you have explicitly asked the authors permission for.
If you wish to use the Sauerbraten source code (ZLIB license), read the src/readme_source.txt file carefully.