ClearTune : Pretty accurate tuner (compared to the korg stdd) and also frequency/note chart (pretty useful)
What's App : Same as Black Berry Messenger on Blackberry
Doodle Jump : Addictive platform game
Gearslutz :
DropBox : File sharing
Not really an Iphone app but works with Itunes
Amarra : replace itunes strange audio engine to have a more pristine sound ( I mean Neutral). Great for comparing sources with Mix/Master