Chromium B.S.U. is an excellent vertical shooter that requires a 3D card that supports OpenGL.Inspired by classic shooter Raptor, Chromium B.S.U. is similarly fast-paced. The graphics are very nice: very detailed ships and backgrounds that are rendered very smoothly without any sacrifice in speed. Ray-traced weapon effects are especially good - missiles leave trail in the air, and explosions "light up" the area for a while. Music in the game is below average, but you can listen to your own music while you play. As someone who only plays an occasional shooter, I find the game reasonably challenging but not impossible or too easy: a good gameplay balance. The game would benefit from more variety of enemy ships (especially bosses), ability to save power-ups for use later, and a buy/sell weapon screen as seen in Raptor. Overall, though, Chromium B.S.U. is still a highly polished, very good looking freeware shooter that you should take a look if your video card meets the requirements.
Note: The download here is the Windows version, so check out the official site linked below for the Linux version and full source code.