Here are links to gallery websites of digital artists I like and to sites which give various resources for digital artists.
Ad Van Bokhoven has some great landscape oil simulations - mainly using Painter.
The Retouching Forum at the Digital Photography Review website is good for all general retouching matters. Not much natural media simulation though.
Another very good place for info on manipulation techniques and processes with a keen photoart forum membership
Phyllis Stewart's lively and friendly forum for all things Retouching and Photoart.
Apart from Craig Mullins, Richard Noble produces the best digital painting I have seen on the web.
Marilyn Sholin's forum for digital painters. The forum is always active with lots to be learned there, particularly about using Painter. Not free, but not expensive.
Demonstrations and Tutorials
Click on the image on the left for a quick walkthrough of the steps I took to produce this picture. You'll need to be able to read pdf documents (with Adobe Reader) to read the document.