Zero-K

Zero-K is a free, multiplatform, open-source real-time strategy game in which massive robot armies fight in an endless conflict. A powerful interface allows players to easily implement their economic, strategic, and tactical decisions. Units interact intuitively, using real-world projectile and movement physics. It boasts over 100 diverse units including Vehicles, Tanks, Hovercraft, Planes, Gunships, Ships, Spider walkers, and dozens of other different robots offering unique strategic options and a dynamic combat style.

Commander wanted! 
Drive giant robots, build an army of a thousand Fleas, move mountains if needed, bury the enemy at all cost!

• Real time strategy.
• Physically simulated units and projectiles.
• Terrain manipulation.
• 100+ varied units with unique abilities.
• Singleplayer campaign.
• Challenging, non-cheating AI.
• Multiplayer 1v1 - 16v16, FFA, coop.
• Multiplayer online campaign.
• Really free, no in-game currency, no unfair multiplayer.