I've been using Pandora for a few years now and I love it. Pandora is like an Internet radio station that allows you to build your own channels based on artists and songs that you like. So, for example, I can type in "Light My Fire" and Pandora will create me a music channel that plays other Doors songs and songs similar in musical style to "Light My Fire."
As Pandora plays each song, you can tell it whether you like or dislike the song it has chosen for you. Doing so, will result in more intelligent choices on its part. I find that after minimal training, Pandora plays almost all songs that I like. It's really very interesting.
I think the coolest thing about Pandora is that it seems to use musical theory and analysis to choose the songs it plays for you. It will actually explain to you why it chose a given song. It will say something like "this song contains soaring lead vocals and rhythmic snare cadences" - or whatever.
You can create multiple channels and switch between them whenever you want. The one thing you can't do is control the flow of the music. You can't choose your songs manually. The only control you have is that you can say that you don't like a song and Pandora will skip it.
Anyway, I highly recommend Pandora for finding new music that you will like.