Ever heard a song that you really liked but didn't know the name or even the singer to help you find it?
With midomi all you have to do is sing or hum the song (even play it with an instrument) and it'll find it. I tried a few songs and it worked great. My choice might not have been the first result it returned, but it was on the main page. No special software is needed, all you need is a browser with Flash enabled and a microphone.
For those interested, the technology behind it is called Query by humming.