Bitfighter – a free multi-player 2-D space combat game with Robotron-like controls. It's a team-based strategy game featuring retro vector graphics and customizable ships. It's fast, fun, and frenetic.
Bitfighter is designed for Windows, Linux, and OS X, and has an active development team. It is open source.
Here's a youtube video - http://www.youtube.com/watch?v=4wQ9U3IUyxk
and more http://www.youtube.com/user/BitfighterVideo