Linux Game Development Center 
Resources for game development on Linux, FreeBSD, and other open platforms.
http://lgdc.sunsite.dkGlutamine: The Game Tool Chest 
C library for networked, 3D interactive applications. Documentation, related projects, source, and binaries.
http://gtc.seul.org/
Simple DirectMedia Layer 
A cross-platform multimedia library designed to provide fast hardware access.
http://www.libsdl.org
Arianne 
Arianne is a multiplayer online games framework and engine to develop turn based and real time games.
http://arianne.sourceforge.net