You might not have heard of Pandora, a personalized streaming music service. The cool thing about Pandora, besides the fact that it's free, is that its music recommendations are based on actual musical analysis (e.g. "vocal harmony, mild rhythmic syncopation, acoustic rhythm piano, major key tonality"), not just artist and genre. Pandora comes from the Music Genome Project.
Well, I made a Dashboard widget for Pandora.
Caveats (not my fault): 1) Dashboard in Tiger doesn't pass events through to Flash applets very reliably (it's a known issue), so it's can be hard to click on things (try clicking in rapid succession). But it plays music fine. It's probably easier to set up your account and music preferences in your web browser first, and then just use the widget for listening to music (think: "It shuffles music for you"). 2) Pandora takes a while to load (like 10 seconds).