Critical Mass (aka Critter) is an SDL/OpenGL space shoot'em up game. It currently runs on Mac OS X, Windows, Linux, and various other Unices.
Storyline: Your world has been infested by an aggressive army of space critters. Overrun and unprepared, your government was unable to defend its precious resources. As a last effort to recapture some of the "goodies", you have been placed into a tiny spacecraft and sent after them.