I lightly modified a version of the Filemon driver from Sysinternals and wrote a little C program that used the driver to monitor for mp3s being played and then grab the perceptual audio hash of the file using trm.exe from Musicbrainz. It then sent the resulting fingerprint off to my website (written in glorious PHP3 no less!) and you could login with an account to see stats on the music you'd been listening to (done with meta data pulled from Musicbrainz).
Surprisingly, it worked reasonably well ...though very sure if I looked at the code now I'd run away screaming.
Really cool to see they're still going strong after all these years!