[archagon]

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

[jcl]

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

http://www.artrage.com/artrage-studio-features.html

[julianwa]

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