Krita is the best and most powerful open source program for digital painting. Period.It has a amazing brush engine. You can use other tool as your brush (for example, clone mode). In this version, the brushes were created by artist like Timothée Giet, Ramon Miranda, Wolthera or David Revoy.
Also has handy tools for painters like rotate canvas, perspective tools, symetric and mirror modes, pseudo infinite canvas, stabilizer helpers, a lot of palette dokers, and now includes some tools for games developers like Clone Array or Wrap Around mode to create tiles.
If you didn't give a try before to Krita, this is the moment. It's one of the best pieces of open source progams out there.