MyPaint and Krita are actually both great programs for digital painting. David Revoy is an artist who uses both (and also created one of the brush sets for MyPaint) and his work is worth looking at: http://www.davidrevoy.com/2-portfolio.html

He has a number of tutorials available on his site too: http://www.davidrevoy.com/4-tutorials.html

I believe he also uses Gimp with the Gimp Paint Studio presets for some things: http://code.google.com/p/gps-gimp-paint-studio/