I have plugged Last.fm before as a great tool for discovering new bands, but it also has another cool feature which is keeping a history of your listening history. And it allows other people to write their own applications to manipulate that data.
For example, here's an application that generates my top listens for the last year months in terms of playing time:
And here's one that generates it in terms of a graph(I'm just showing part of it cos my entire history is too long):
The cool thing about the 2nd application is by looking at it I actually get reminded of what happened in the past. For example the huge spike for the artist The Roots was when they came over to Singapore for a concert. Another memorable one was Opeth, I could exactly pinpoint the exact time when I finally realised the brilliance of their album Ghost Reveries and started listening to it incessantly.
To a casual listener it might not offer much purpose, but to anyone serious about their music, it's a great way to track how your listening preferences change over the years.