Saw this neat little game mentioned on Rock Paper Shotgun.
Gimbal is a 2D team-based multi-player space shooter where you design your own ships. Ship designs are stored as PNG images, meaning you can easily share them around (you also steal the designs of ships you defeat). The game supports 64 players, and it even includes bots and a dedicated server.
Anyway, there's currently a "Pay What You Want" deal for it running over on IndieGameStand, meaning you can pick it up for as little as 25 US cents.