this is awesome.
the infinite jukebox.

its created with the echo nest api.

from their faq:

How does it work? – We use the Echo Nest analyzer to break the song into beats. We play the song beat by beat, but at every beat there’s a chance that we will jump to a different part of song that happens to sound very similar to the current beat. For beat similarity we look at pitch, timbre, loudness, duration and the position of the beat within a bar. There’s a nifty visualization that shows all the possible transitions that can occur at any beat.