Bitfighter -- the fast-paced team-based outer-space multi-player arcade game. Blast your friends, zap your enemies. Steal their flags and nuke their cores. Customize your ship to be sneaky, or overwhelm with superior firepower. It's fast, fun, and frenetic.
Bitfighter is free. Period.
Bitfighter runs on Windows, Linux, and OS X, and has a lively community and an active development team.