I've tried the opensource software before and was disappointed, but that was a couple years ago now. I must admit that many of the free alternatives are getting to be extremely well worth looking into.

Gimp Paint Studio (Not the standard Gimp)
[link] (the files you need to add to gimp)
[link] (the instructions on how to set it up

MyPaint (Great alternative to Painter)
[link]

These are available for both Windows and Linux. There are some free 3D programs as well, but everyone knows about Blender and most likely knows of Wings3D too.