Last.fm Explorer was created by Alex Pretzlav for cs294-10 at UC Berkeley. You can read about it or get the source. Last.fm Explorer is powered by Flare, Flex, and of course Last.fm.

Clicking the "Tracks" drill-down level generally requires a large amount of computation and can lock up your browser while the application is rendering.
The line graph view can be very slow, particularly if there are a lot of items being displayed.
No progress is displayed while loading tags or rendering the initial visualization.
When viewing two users, if one user did not submit any tracks for weeks on the head or tail end of the last 20 weeks, the two users' graphs will be misaligned temporally.
There is no way to remove a graph in the comparison view or to change which users are being compared.

Last.fm Explorer