Psykopaint (Web Applicaion)
Description:
An online "natural media" application which extracts the colors from a base reference image and allows you to paint over it, using distinct "onion skin" layers.
The website is nicely presented. Very easy to apply free-hand brushstrokes — but people with actual experience in painting will benefit the most.
Link to Online Application:
http://www.psykopaint.com/
Image(s):
(Photo of 3D model from: Google's SketchUp 3D Warehouse)