Mirror's Edge. While I don't think it is directly underrated, more like overhated. I love that game for bringing awsome first person gameplay. I didn't care a second for the short and boring story, all I wanted was the gameplay itself. Mirror's Edge is the only game I can ever remember doing all the time trials as fast as possible.
also I got the Platina trophy
Also i just have name an older computer game: The Movies. It is an awesome Hollywood-ish tycoon game where you make your own studio and fill it with actors, directors, scriptwriters, scenes and so on. The game was just to awesome! Gotta go play it again!