Echo Nest Remix lets you remix, re-edit, and reimagine any piece of music and video, automatically and algorithmically.
Remix has done the following: played a song forever, walkenized and cowbellized hundreds of thousands of songs in a week, reversed basically everything, beat matched two songs, split apart DJ mixes by their individual tracks, made new kinds of video mashups, corrected sloppy drumming, synced video to a song, transitioned between multiple covers of the same song, made a cat play piano, and taught dogs to play dubstep. Check out all the examples here.
Remix is available as an open source SDK for you to use, for Mac, Linux, and Windows:
Install for Python:
sudo pip install remix. Full installation details, packages for Mac and Windows, and complete Python documentation are here.