If you haven't checked out Pandora
it's worth a look. It's basically a create-your-own music station (or 100). For starters, plug in the name of a favorite musician/musical group. Pandora matches the characteristics of the artists songs with other like-songs and begins building your station based on their detailed index of songs by musical similarities and feedback you provide along the way.
Nice, friendly, simple user experience:
It even lets you easily share your stations or listen to other most commonly shared stations.
A nice alternative to listening to an mp3 playlist, tho' not as portable.