Shooters... hm. Can't think of any.
I do, however, play an RTS or two. Sci-Fi?
OH WAIT A SECOND.
XBlaster, its a browser game, or it was when I played it last in 2008 XD
You control a mech, its an MMO unless you do missions. You can buy some pretty freaking awesome weapons, too. You can play for free, and still have a great experience, but if you pay you get extra points and such to unlock things quicker. Either way, you still unlock them eventually.