OpenFrag is an Open Source, multi-platform game. It plays in a medieval environment where Orcs and Humans fight to the death in a multiplayer session. Compile instructions and dependencies can be found on the following wiki page: http://wiki.openfrag.org
NiL is a modular multi-purpose gaming engine including a multiplayer worm game a little like quake, except it's 2d and a little like worms except it's realtime.