Phil, great article. I am both surprised and happy tuxpaint was able to pull them in to that extent. My daughter used to use that program (from about age 5 to 8) pretty regularly. She has since moved on to 'playing' with scratch, which continues to push her creativity.
Another game that is simplistic, but has levels from trivial to very challenging, is magicor (and to be honest, one of the only computer games to grab my interest). It is more of a problem solving game (the penguin pushes a block of ice to extinguish fires). The graphics are simple so it will run on any hardware, and there are enough levels to keep any person satisfied for a long time. It appears to really get their interest when they challenge each other on a given level.
Thanks Logan, I was not aware of these projects. Very clear article.