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.
Unvanquished is a fast-paced, futuristic FPS with RTS elements, pitting technologically advanced humans against hordes of highly adaptable aliens. The player can choose from either team, providing a fresh gameplay experience on both sides of the conflict.
The game is under highly active development with new releases frequently published.
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 Open Source 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.