I finally got around to checking out Diaspora: Shattered Armistice the other day. It's a free game that basically gives you a good ole' X-Wing vs. TIE Fighter style flight simulator set in the Battlestar Galactica universe. Download is about 1.3GB, and it support Win/Mac/Lin.
The good stuff: It's got both single player and multiplayer modes. They obviously put a lot of thought into the game and there's a pretty good amount of detail. Combat is hectic and "realistically fake" like the old X-Wing games were (ie, you let off the throttle and you stop moving, despite the fact there's no friction in space).
The bad stuff: It's hellah complicated. There are a lot of different buttons to push, not all of them make sense, and not all of them have anything close to an easy to remember mnemonic. I'd also argue that it suffers from ARMA II's problem of having too many buttons to press and some of them could probably be merged into one key. The craft are *very* responsive, which is cool, but in order to get all 6 degrees of control you need both the mouse and the keypad which makes flying kind of awkward. It almost feels like you're trying to fly emacs.
As for the production values, they aren't bad for a free game. The scripts for the voice overs are actually pretty cleverly done to avoid that awkward "Anonymous rookie pilot, stop those planes!" situation.
If you're looking for a complex combat flight sim that has a few rough edges, this might be right up your alley. If you're looking for a quick, cheap yank'n'bank shooter thrill to blow off some steam, this probably isn't for you.