When I listen to music on my computer, my music player automatically submits every song I play to last.fm, which keeps track of everything. This means that I can see on my profile what my favourite artist/song has been for the last week, or ever, and so on. It can be quite disturbing to have conclusive proof that your favourite song is not what you thought it was, but it's interesting nonetheless.
Not only is it interesting, however, it is also useful. The site has a very useful and clever feature: it can suggest new music to you that you will like. Since it has complete information about the music history of all of its members, it can compare your listening habits with everybody else's and make comparisons. If you listen to 90% of the same music as somebody else, your musical tastes are probably very similar - the site can thus suggest that you try out some of the other music that the other person listens to. Scale this up to thousands of people and thousands of songs, and you get a remarkably accurate method for gauging music taste.
I'll leave it to you, if you're interested, to find out the other nifty features of the site (it streams music to you if you want, for example). What I wanted to share right now is lastgraph.
I have been submitting my music (on and off, mostly on) since April of 2004... I can now use lastgraph to draw a graph of my complete listening habits. Which I did. And you can look at it, if you want. It's available for download here in PDF and SVG forms, or you can see a PNG rendered version of it below (click to enlarge).