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So hard to pick just one..

pandora is amazing.
shazam is possibly the coolest thing I have ever seen.
tap tap is actually a lot of fun

I also find amusement about all the little entertaining apps like...
ipint - always the best to show your friends
flashlight - i love the strobe setting, especially because you can change the colors
phonesaber - fun for any star wars fan

I found aim a little disappointing actually. especially because it's glitchy
 
A very often overlooked program: Graffitio.

Being able to leave messages pinned to a physical location is greatly amusing. Like having invisible message boards wherever you go. I'm having way more fun with this program than with any of the games I've tried.
 
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