Hello, I'm the author of Rendera, a free (open-source) painting app. I recently rewrote the entire thing from scratch, and you can get it here:
http://rendera.sourceforge.netI made this mostly for retouching, but have done game art also. At the core it's like a fancy version of MS Paint.
The current todo list:
-copy/paste/clipboard etc
-patterns
-isometric grid
It's difficult as a developer to write useful software without feedback, so that's what I'm after. (Also will give credit for ideas/code contributions.)