Have you ever tried GPAL?

It is somewhat similar to Pandora.
Works with itunes - type in an artist and it creates new playlists from your library.
But it also finds and allows you to add music from new artists. (For some reason these show up under my podcasts.)

I haven't used it much recently so I can't recommend anything specific, but if you are an itunes/ipod user you might want to take a look.