Almost all of my recommendations (Mercenaries, Ratchet and Cank, SOCOM, Army of Two and even Metal Arms) were already mentioned.
Jet Force Gemini is a fun game but it does show its age I will admit.
I've actually been playing some pretty cool demos recently, Red Faction Guerrilla is absolutely funtastic, looks to be my personal game of the year, and everything the second Mercenaries SHOULD have been. The Multiplayer demo for Bionic Commando is up and it plays really well, once you get used to the controls.
Crackdown is an underappreciated gem and should be really cheap now. there is something just magical about jumping between buildings raining death from above upon the ususpecting thugs. Plus you can kill people with a RUBBER DUCKY, that should be proof enough of its awesomeness.