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.
Da Vinci Wars is a multiplataform OpenSource RTS (Real Time Strategy) multiplayer game based on the 16 century Italy, with the aim to emulate the battlefield behavior of most Leonardo Da Vinci's designs.
Tactical Warfare Engine is an OpenSource 2.5D Real-Time Strategy game engine made using OpenGL. It aims to be a replacement for the aging StarCraft game engine, but also supports the creation of entirely new RTS games.