Last Genius is a hack I put together for Last.fm's Hack Day in December 2008. It creates Genius Playlists from songs on Last.fm and plays them for you. I wrote a blog entry about Last Genius and the code is on GitHub.
How to use Last Genius
Drag the Last Genius bookmarklet (that link just back there) onto your web browser's toolbar.
Visit a song page on Last.fm - for example Longpig's "She Said".
Click on the bookmarklet. A pop-up will appear and a few seconds later music will start playing.