YouTube / Last.fm Mashup
Many artists and labels post official music videos online. This YouTube/Last.fm
mashup will try and play back the videos to the albums and songs you request. It does that by searching Last.fm for music data,
and at the same time searching YouTube for appropriate videos.
Unfortunately, the whole thing is built using Google's toolkit for webapps,
is also running on Google technologies.
For what it's worth, here's a screenshot of it in action: