Armored Core 4-- I lurved that game, and was hoping to enjoy zillions of hours of frantic online robot-on-robot action. But no one plays online, so my awesome AC and I are all alone.
Jak 2 and 3-- The Jak series doesn't get nearly enough credit for its awesomeness.
Timesplitters: Future Perfect-- Man, what I wouldn't give for Timesplitters to replace Halo in popularity.
Killer7-- Gamestop had, I shit you not, an entire row of Killer7 games on display because no one ever bought them and they were taking up space in the bulging PS2 case. I happened to have 11 dollars so I bought it, quite enjoyed it.