AudioScrobbler was a site which tracked what music you had listened to, and gave various statistics based upon it. They've now become Last.fm and their statistics have improved a lot.

Now I can see what songs I've listened to the most in the last week, or, even better, what I was listening to in any particular week.. so I can see I had a Fiona Apple fixation in the first week of April. The perfect tool for us music geeks.

I've also used it to find "similar groups" to bands I already like which has led me to bands like Frou Frou and Fall Out Boy. There's even a streaming radio station which plays music specifically selected to fit your musical tastes. Very sweet.