ArtRage actually has a free version on it's website! It doesn't have all the features, but it should be enough to try it out, at least.

I have the free version and okay, the lack of layers function (You get it on the paid one) annoys me, but apart from that slopping paint around and playing with the chalks and pencils and felt tips is kinda therapeutic! :D

EDIT: Ah! I found the pade with it on! Here you go!