List of some of my favorites:
OMG! Pirates! - Sidescrolling fighter with combos, counters, and special moves.
Minigore - Isometric horde shooter.
Rock Band - Suprisingly fun (although vocals have been reduced to buttons only) with WiFi and local Bluetooth multiplayer. Music Store with DLC on the way.
Zenonia - Kind of like Zelda with leveling, skills, quests, and a Good/Bad system.
Peggle - Because it's ♥♥♥♥ Peggle.
Worms - See Peggle. Substitute "Peggle" with "Worms".
Duke Nukem 3D - The controls are a little odd, but you get used to them.
Call of Duty: World at War Zombies - Fun FPS/Zombie Defense, WiFi and Bluetooth (Only one level apparently, supposed to be more).
Knights Onrush -
Defend Your Castle style game with upgrades.
Katamari Damacy - A bit of lag from time to time but plays a lot like the console versions.
Song Summoner: Encore - Turn-Based Strategy that uses the songs on your device to summon monsters.
More with less detail:
Black Shades - Colosseum - Inotia 2: A Wanderer of Luone - Monopoly: Worldwide Edition - Steam Wars - Speed Forge Extreme - Stone Wars - Sonic the Hedgehog (Genesis) - Super Monkey Ball 2 - Crystal Defenders
And some I haven't gotten to try yet:
Command & Conquer - Assassin's Creed (and the sequel) - Need for Speed: Undercover/Shift - The Sims - Max Injury - Dismount