New software accompanies any melody in any style
Researchers at the University of Southern California have created a piece of music software able to analyse music by an artist, then create accompaniment to any melody in that style.
According to its creators the software prescribes chords at checkpoints where it finds the harmony unambiguous, and uses neo-Riemannian transformation to build chord sequences between these points.
It might sound complicated, but apparently the software is simple enough to run on a regular PC, and will likely be eventually implemented in Java or C++.
Check out this article for more info, and see the University of Southern California website for some examples of the system in action.