Combat Simulator Project r2280
A free, open-source combat simulation.
Apart from the usual OpenAL/OpenGL/SDL tricolore and general system libraries, the CSPSim engine depends on OpenSceneGraph, osgAL, Python, SWIG, Vorbis and probably a few more.
For now, the available content includes only a few models, airports, and the Balkan terrain which all has to be thrown together and built. Just follow the white rabbit in the instructions linked below.