Asteroid Outpost is an open-source, 2D, coop tower strategy game written in C# using the XNA framework. You can currently build solar power stations, connect your structures using power conductors, build laser miners to mine minerals from the asteroids, build laser towers to defend yourself against the AI, and upgrade your structures. Multiplayer is still a little rough, but there's a server browser and two players can play attempt to hold off the AI together. In both singleplayer and multiplayer, there in an "Endless" mode that causes an ever increasing number of AI drones to attack your base until you eventually fall. You can watch me doing exactly that right here:
All of my bugs and tasks are being tracked in Trello: a collaborative tool that is really light-weight and free from rigid rules. The source and releases are available on Bitbucket, so feel free to poke around.