Space Exploration: Serpens Sector is a very long-running project to make a space exploration game in the vein of Strange Adventures, Star Control, and others. It's gone through a number of iterations and refinements over the years. In its current state, it is now gameplay-complete. The gameplay focuses on making interesting decisions balancing risk, reward, limited resources, the morale of your crew, and your reputation with the alien factions of the sector. The gameplay does not focus on combat.
MV3D is a virtual world and multi-player game framework for use with Python. It was designed with scalability in mind and is able to distribute a world across as many servers as needed while dynamically balancing the load. The simulation framework is not specifically slanted towards any one genre of online game or virtual world, and can just as easily be used for a space game as a fantasy setting. Objects on an MV3D server can be simulated using the ODE physics engine for realistic interactions. A single server is able to host thousands of of simulated objects. The client works with both the Ogre3D and Panda3D renderers.
Sauerbraten (a.k.a. Cube 2) is a free multiplayer/single player first person shooter, built as a major redesign of the Cube FPS. Much like the original Cube, the aim of this game is not necessarily to produce the most features and eye candy possible, but rather to allow map/geometry editing to be done dynamically in-game, to create fun gameplay and an elegant engine.