One of most intriguing features of Musipedia is its Keyboard Search. You can search for music actually using the notes on the piano to guide your search - this is the first time I've ever seen anything even remotely like this and I just thought it was way cool.
In addition, you can search for music by whistling a tune (no joke). From the site: "The applet at the bottom of this page can analyze your whistled input and calculate the Parsons code, which you need for a search. For some people, singing works better than whistling. It is also conceivable that certain musical instruments work still better."
Related: If you're looking for multimedia on the Web, check out my list of the Top 10 Audio Search Engines-BitTorrent Search Engines, Free Movie Downloads, Videos.