Andy at TechDigest has put together a top 10 of music apps for the iPhone here - http://www.techdigest.tv/2009/07/top_10_iphone_m.html - And while it is a wonderful list it misses for me the number one which is Simplify Media http://www.simplifymedia.com/
I have been using this for a few weeks now and would def say it is my fave iPhone app. It basically allows the user to stream music from the hard drive of their PC remotely to their iPhone. So instead of being limited to the iPhone's built in memory you can hear whatever you want when you want. Well, that isn't strictly true, you need to have a Wi-Fi connection, but it is still a genius app.
Of course the ultimate iPhone music app will be this one http://www.techdigest.tv/2009/03/spotify_coming.html
