I found a cool game called Astro Battle. You can build your own space ships using different parts and fight other people's ships with it online. It's really cool because unlike other games, everybody's ship is different :). Unfortunately, it's not really free - if you subscribe, you get more money to build ships. You can play as much as you want for free, though. I decided to share it because they don't seem to have many players.

www.astrobattle.com

*kills hitman*