Ironclad: Steam Legions is a steampunk strategy game with an AI opponent. It lets you pit your strategic skills with battle tanks, infantry, artillery, and warships against a fast thinking enemy in a hex-based battle arena. It is still under development in the alpha phase. However, the game is fully playable, with a complete set of tutorial levels to master, with sound effects.
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.
Liquid War 6 is an original multiplayer wargame. You have to control an army formed with thousands of pixels that look like liquid, and try and eat your opponent. This is a total rewrite of the (now quite old) Liquid War 5 version. It's still in beta stage, with many imperfections and no network play but it's playable now.