Angry Birds, Cut the Rope, Words with Friends, Fruit Ninja..
Those are my 4 favorites.
Words with friends especially. If you have friends you can play with it's awesome, but even if you don't you can match up with random people and have a blast too... It's basically scrabble over the internet. Pretty fun and challenging...
If you get it it's 99 cents, or free (only difference is the free version you have to put up with advertisements after every turn). Also, if you get it shoot me a game invite. My user name is the same as here: DTP88