MySong, a project I worked on last summer with Dan Morris and Sumit Basu of Microsoft Research, seems to be hitting the blogs:
http://www.boingboing.net/2008/03/03/microsoft-researchs.html
I'm a bit surprised at the negativity of some of the commenters. Remember: this isn't supposed to be the ultimate solution to music composition. The chord progression model is pretty simple and the synthesis (at least without Band-in-a-Box) is just a Let It Be pattern. What MySong does do, however, is let people who have no idea how music "works" play with basic composition. I know a fair amount of music theory and still had tons of fun singing into MySong and messing with the chords that came out. In fact, I spent more time doing that than actually working on the code (but don't tell anyone that).