GTC 0.8 Release
Version 0.8 of the GTC (Game Tool Chest) is now available, a GPLed library by Sébastien Loisel. Here's a description of the lib from the GTC web site:
GTC is a C library for making networked 3d interactive applications.
So far, the only really good part is this. You give it a scene graph (see below) and it will make sure that all clients connected to the server have a synchronized copy of the scene graph. For instance, you could have a scene being a maze with people shooting and all the projectiles, with the clients being players wanting to see where they are. Networking is handled for you, the encoding is compact. "