Hacker News Reader

Corel Painter has paint mixing, but I don't know how it compares to Paper's approach.

ArtRage also has a paint layering/mixing model, although I'm guessing it's using neither Kubelka-Munk nor Paper's survey-based approach.

Krita has (or had) an interesting implementation for paint mixing that uses Kubelka-Munk: http://www.youtube.com/watch?v=lyLPZDVdQiQ