Yet another cool tool for music discovery: OWL. Pick any MP3 file from your collection, upload it to OWL, pick the bit of the track that makes it special to you... and it will search it's database of Creative Commons shared works, and recommend music with similar sounds. I tried two tracks and was very impressed by what it found -- some of it was stuff I already knew I liked, but there was a lot of stuff I hadn't heard before.
Get an account before you upload your music. The process of uploading and picking the bit you like takes a bit of time. I wanted to start organizing my search results right away, but I couldn't because I hadn't signed up yet.