I’m pretty enamored with Pandora.  This site allows you to build an internet radio channel based on songs and artists that you like.  Backing this is a database called the Music Genome Project, which should obsolete the “genre” tag on your MP3 files, which lack the consistency and taxonomy to be really useful.  Instead, the Music Genome Project classifies each song and artist based on the vocal style, tempo, key, etc. and looks for music that matches those values.  I think that Pandora could likely become the next CDDB, with music players leveraging this database (written into MP3 metadata) to allow me to select a song on my Zune and then have it play a list of other songs like that one.

Here’s a sample station seeded from a single song.  I like this song for stretching after I exercise and found that Pandora did a pretty good job of lining up similar songs.