Play virtual DJ with two youtubes and a... umm... crossfader
In case you didn't get it from the domain name, the web site basically presents you with two YouTube videos. And a crossfader. That's about it.
Slide the fader left or right to adjust the volume levels so that one video becomes more or less audible. Theoretically, you could also use the site to say, see what Axel Rose would look like while singing Tom Jones cover songs.
You can use the search boxes below each video to find new YouTube videos to try out. Of course, the biggest problem with searching randomly for audio to create a mashup is that most of what you find won't be in the right tempo or key. There's no pitch or time control functionality on the site. Just two YouTube videos. And a motherfucking crossfader.