So i use iTunes as my music player app. I downloaded the new one, version 8. This Genius setting it has is well... genius. While i do make mixed play lists, I've always had a hard time just downloading one song. I listen to albums. What this means is i have gigs and gigs of music on my hard drive that i really haven't listened to. Now you can make smart play list to listen to them in a rotation but that is using the tags and on any rock album you might have fast and slow songs that work for the album but not for a mix. This Genius play list is taking your song information and others, correlating it then playing other songs from your library that match the song you picked. it's very, very good.
There is a kind of Skynet feeling about the whole thing though.