last.fm ... musical station for finding some new music. you can add friends as well and then listen to the stations that they like or compared by things they listen too. http://www.last.fm/user/Travace
EveryTrail... a GPS enabled app that allows you to record a ride,hike, drive or whatever and save it as a file that is Dlable to other GPS devices. It has all kinds of recording features. it also links to a site. http://www.everytrail.com/
Shazam... app that allows you to record a part of a song and it will give you all the info of the song. Very usefull for when you hear a new song but dont know what it is.
i dont have a iPhone anymore but when i did those were three that i used a lot.