Pandora® internet radio Pandora is the best thing that ever happened to me. This app lets you teach it what kind of music you like and then plays you radio stations with that kind of music. Highly customizable and the mobile app works great!
Shazam Gone are the days of trying to figure out what song you just heard. Shazam is a song identifier and will help you build your music collection by introducing new artists and tracks!
Spotify This app is amazing in the sheer number of songs it makes available (15 million) to you over your phone. Listen to either tracks stored on your phone or streaming from the internet.
Last.fm This app streams a large collection of music to your phone. Also find out more about musicians and track local concerts.
Android Music Player Just a simple, decent music player - but sometimes that is all you need. Doesn't take up much space.