Rubato Composer is free (GPL) software that allows one to compose music or transform existing music with the help of mathematical Category Theory and Topos Theory. It is currently the only software for music composition based on Category Theory. It is being developed by Gérard Milmeister and Guerino Mazzola, both of whom are currently working on a book about it, The RUBATO Bible (working title). The book is currently in preparation, to appear during 2008.
The only other similar software, Presto, is no longer developed and runs only on Atari computers.
References
Rubato Composer Wikipedia(Text) CC BY-SA