Wheels Of Steel - Browser-based turntable mixing simulator
The above video is a demo of what it does, but you can try it out (on modern browsers) here:
There are even more features if you have Safari 5, Chrome 12, MAC OSX, as you can access ‘scratch mode’
As far as I can see at the moment, you can only use selected tracks, I cannot see any option to use your own mp3 collection or tracks stored anywhere.
For the web-technical amongst you, how this was put together can be found in this blog post (scroll down a little bit)
If you want to hurt your ears, here is a video using two sources of the Nyan Cat meme