Just curious, I don't have time to look through the app store, there seem to be 100's out there, tell me what you like!
One I do have is Shazam - if you hear a song you like on the radio, hold your phone up and the app finds it (if it exists) in iTunes
Well I'm a bit biased but I like FlipFrog (http://tinyurl.com/ce3ano) for the kids seeing as how my husband made it for my daughter :) but also Shazam, WeatherEye, Google Earth, Connector, Ocarina (my daughter loves that one too though she blows spit all over it!) and a bunch of others. My iPhone is kinda cluttered :)
Shazam is awesome! Ambience is good to mellow out.
Honestly, the most useful thing I find is plain ol' Google Maps. I searched and searched for a coffee house finder and didn't really like anything, especially when you're trying to see if there's a Starbucks a long the way when you're taking a road trip. So much easier to just put in Starbucks into Maps and look for the pins.
OK - I don't have an iPhone, but my dad does and I love the "lighter" application so if you are at a concert...you can wave it in the air. Hilarious!!! I love it almost enough to get an iPhone! www.zdaisy.com