I Am T-Pain - seriously it's hilarious. Not really a game but more of a karaoke autotuner that has nothing but shitty T-Pain songs which makes it even more ridiculous. Very fun to have around with your friends.
Space Invaders Infinity - absolutely awesome. Don't let the Space Invaders name fool you. This is a SHUMP.
The 3D Rollercoaster games. I think there's about 3 or so of 'em and you can buy it in a bundle. Surprisingly entertaining.
Space Miner: Space Ore or Bust. It's like Asteroids mixed with an RPG and it's one of few "dual stick" control methods that actually feels decent on the iPhone.
And I like Virtual Villagers 2 & 3. It's an acquired taste because you're not really directly controlling your tribes but just making sure they're staying on task. Your people live, have babies, die, research technology, build homes, etc. all with the purpose of accomplishing a set of riddles/goals that you need to finish to "win". These games take weeks to finish but luckily they also run in the background so you don't have to keep watching them. Ignore them too long though and you could end up with nothing but corpses.
