M.A.R.S. - A Ridiculous Shooter
Even though I'm no fan of the multiplayer (zero-)gravity-shooter genre, I enjoy playing M.A.R.S., which features AI enemies and wingmen, various weapon types, upgrades and a pretty cool game mode, where a huge cannon will shoot at the planet of the enemy team of whoever is holding the remote to it. I also think it's darn pretty.
BAM!
M.A.R.S. uses shaders (but does not require them). The game menu is well-made in both look and feel too. The media license is not specified. Code is GPLed. M.A.R.S. runs on Lin/Win (get it here) and uses the SFML library (2.0, I had to get it from SVN).
BOOM!
If you want to help the project out, you can do so by translating it. English and German translations are available so far. Since SFML 2 has recently been ported to OS X, it probably would be welcome if someone could provide a build as well.