Chaotic Rage is a crazy top-view shooter! It has zombies, tanks, destructible environments, many weapons, multiple maps, and is moddable!
It's a simple shooter (think Worms or Team Fortress 2 rather than Call of Duty) but very fun. I'm always working on and improving the game, with (usually) fortnightly releases. Anyone can make improvements through the mods system or by playing with the code on GitHub.