Science Daily has an interesting, if a bit wide-eyed, look at ASSA (Automatic Style Specific Accompaniment), computer software that applies concepts of music theory to intelligent automatic harmonization of melodies.
In a nutshell, the software harmonizes melodies in the style of U2, Radiohead or other existing source material.
The system is described in Ching-Hua Chuan’s paper on Automatic [...]

Antares has announced the release of Harmony Engine, a Vocal Modeling Harmony Generator, available as RTAS (Mac and PC), VST (Mac and PC) and Audio Units (Mac).
The Harmony Engine is a real-time harmony generating plug-in. With four independent harmony voices, a variety of powerful harmony generating modes, humanization features for natural sounding performances, and a [...]

 

 Yesterday - dry [0:50m]: Play Now | Play in Popup | Download

 

 Yesterday - harmonized [0:49m]: Play Now | Play in Popup | Download