From Paul Lamere:
The Infinite Jukebox is a Web application that lets users produce "never-ending" remixes of any song. Users can upload MP3 files to The Echo Nest, where songs are broken down into individual beats, which are then analyzed and matched to other similar sounding beats. For beat similarity the software looks at pitch, timbre, loudness, duration and the position of the beat within a bar.
The matches are then used to create many different paths through the song using similar sounding beats. As the song is played, when the beats have similar sounds there's a chance that playback will branch to a completely different section of the song. Ideally, since the branching occurs among similar sounding beats, listeners won't notice the jump. The process of branching can continue forever, giving users an infinitely long version of any song.
An attractive chord diagram shows the beats of the song along the circumference of a circle along with with chords representing the possible paths from each beat to its similar neighbors. When the song is not playing, you can mouse over any beat and see all of the possible paths for that beat. When the song is playing, the visualization shows the single next potential beat.