Gratuitous Space Battles. I bought it on Steam on a recent sale and have been having a blast with it.
I also played through Crysis, and halfway through Crysis: Warhead. It took me a while to warm up to Crysis, but I ended up loving it. Especially the second half of the game, once you were fighting aliens. So far, I’m loving Warhead even more right from the beginning. They improved the controls with the keyboard and mouse, and they have a much better controller scheme, and the game’s level design is better this time around too, since they tend to take you straight from action piece to action piece.