In the spirit of Valentine's Day, and by that I mean "this doesn't have anything to do with Valentine's Day," I present something I love:
Open data formats.
Most of you know I've been logging my listening habits over at last.fm for several years now. Some time ago, a nifty gentleman wrote a set of scripts (available at lastgraph.aeracode.org) to generate a visual representation of a single user's listening habits over time.
I think I have earlier data saved on files at home, but I'll have to check. Clicking on the files will take you to the larger versions archived on flickr.