gosh...I can't think of anything I don't like.
But here are some things I had no idea I would use that I really like....
AP news app - FREE
Google feature - use this all the time when I am on the road and need restaurant info, etc. - comes on the phone
Games to play when waiting - I play the free ones
ability to save webpages to my iPhone home screen - so I can check on blogs that I follow by just pressing the icon
Skype - I use this to call my friends that used to be a long distance call and use up my minutes - so I dropped my minutes on my phone and am saving a little money
I use the camera to "tweet" pictures. This is fun and my friends have enjoyed it.
MAPS feature has gotten me out of a few tight spots when traveling to other cities! It even looks things up by "name" and not just the address. The GPS feature allows you to track your movements on the map so you know when to turn, etc. (Of course, you should let someone else use it while you drive.

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Weather - I have used the radar a lot! I use the WeatherBug Elite app (I think it cost me 2 bucks?)
WDW apps will be fun to try when I am there this week!
I use it for exercise apps - never thought I would do that.