[ndc]

It's odd that nobody has mentioned Pandora yet: http://www.pandora.com/

Put one song in, and it will give a set of songs that are similar to your song, with explanation why it thinks they are similar.

Too bad it is available only if you are browsing from US IP address.

[randallsquared]

I was going to mention it, but then he specifically asked for things outside his current tastes, which preclude putting something that he likes in Pandora and expecting something really different. It is true that Pandora sometimes comes out of left field with a track that's nothing like what I've been listening to, but I think they consider that a bug rather than a feature. :)