0 A.D. is a free, open-source, cross-platform real-time strategy game.
0 A.D. (pronounced "zero ey-dee") is a cross-platform real-time strategy (RTS) game of ancient warfare. It's a historically-based war/economy game that allows players to relive or rewrite the history of ancient civilizations, each depicted at their peak of economic growth and military prowess.
The OpenRTSS project aims at producing a free game engine for making "Real-Time Strategy Shooters". The engine is written using Unity for simplicity and fast development.