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