I've been a user of Pandora for a long time now. And it just keep getting better.
If you're not familiar, it is a music service (free) that allows you to find music and listen to similar songs based on the 'genome' of the song (a number of factors are used to build the 'genome' of a song, and the service matches other songs with similar 'genomes').
The reason why this service as become my favorite app of all time is the iPhone Pandora App. It has replaced my satellite radio for my main source of music of late. Even on the GT, the iPhone's EDGE service was able to play an uninterrupted set of songs on the move. It didn't work really well in remote areas of Utah or Colorado, but I guess that's what the iPod feature is for.
When you are in a 3G (or 2.5G EDGE) coverage area, music is downloaded and played on your iPhone (or other smartphones that have a Pandora App). This is way cool stuff.
Pandora has become so popular that the greedy record companies are trying to squeeze more money out of Pandora as an internet radio station. Our lovely Congress recently passed legislation that increases the royalties paid by internet radio to be more than 4 times what terrestrial radio pays and double what satellite radio pays. Amazing what lobbyists can do. Can't we make them illegal (but that's for another discussion).
Anyway, while it lasts, Pandora is my favorite app of all time (until another one comes along