This is something I gave a hard time with quite often. This post is as much for me in the future as much as it is for all of you.
So I'm sure you all know about Shazam now. I believe that most smart phones come with it as a default app. Basically, when you hear a song you like you start the app and it tags the song telling you the artist, song name, album, etc. You can also buy the song directly from the app.
Just like iTunes Genius, it tells you what you aren't listening to that you should be based on what you ARE listening to.
Now, this one I really liked. Its not as fancy as other sites, but it does the job well. You enter three artists you like and it gives you one suggestion based on those. You can then either click on "I like it" if you've already heard of this artist, "I don't like it" if you happen to hate them, or "I don't know it." These three options all lead to the same destination: Another suggestion.