Pandora is cool, but I find their playlists can be a bit repetitious. Right now I'm loving Songza, another free music streaming service. Songza helps me set the mood right by suggesting playlists by activity, for example "Hosting a Sexy Pool Party".
Audio files take up lots of space. If you don't need or want a vast portable music library, here's how to change the default setting and limit how much music is synced to your iPhone.