I've known Animusic for a really long time (in fact when I was studying CGI Animation, I wanted to do a similar project). Here are a couple of my favorite pieces.
Created by Microsoft in 2009, Songsmith is a software that lets you record your own voice, and it will create a musical accompaniment. It didn't take long before people tried applying it's genious algorithm to famous musical pieces. Here are my favorites.
David Cope is a music theory teacher at the University of California (at Santa Cruz). He created in the 1990s advanced software to help him compose music, which gave birth to Emily Howell, an artificial intelligence able to compose music on her own.