The more and more removed I am from playing this game, the more and more I enjoy and appreciate the quality. I'm glad Rockstar took a chance in this detective-type game. Extra bonus for the cars available to drive. I don't think this game qualifies as "under appreciated" but I'm not sure it got the attention it deserves. Guessing it also won't see a sequel.
Sonic & Sega All-Stars Racing
What a great Mario Kart game. No really. Only it's better than Mario Kart Wii. Great tracks, great racers, and great multi-player (though I only played online races). The only game I have ever intentionally gone about getting all achievements. Have yet to play the sequel, but hope to find a cheap copy one day.
& Picross 3D
Two excellent puzzlers. Hope to see more sequels (though probably won't). For more of my thoughts, you can read my review for GameCritics.com
Just a few, may post more later.