Open source clone of the Age of Empires II engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
A fantasy real time multiplayer strategy game with player scripted AI
A mobile tower 3D tower defense/defend the flag game
Cross platform 2D game creation framework with Python bindings
City-building game that will contain the best parts of SimCity.
Wrote in c++ and using SDL. V0.02a