Started in January 2000 (around the same time I was trying to figure out whether Natty Light or Natty Ice was better in the second semester of my sophomore year of college) a Stanford graduate launched the Music Genome Project that would attempt to quantify the "genes" of songs and music genres by incorporating factors ranging from "gender of lead vocalist, level of distortion on the electric guitar, [to] type of background vocals." This methodology was adopted by another website called Pandora which utilizes the Music Genome Project's data to create an Internet radio station that dynamically interacts with your musical preferences to automatically create an ongoing playlist that plays songs that it thinks you'll enjoy based on your preferences. You are given the option of giving each song a thumbs up or down which helps the site hone in on what you like.
It's a really terrific service that uncovers a lot of music in a genre you might not know that much about. For example, I typed in "Esbjorn Svensson Trio" and as a result I've been listening to some really terrific jazz tonight. I suggest you do the same.
(Thanks Nicole!)