This page is OUTDATED. Go to the new mypaint homepage instead.
Wetdream (server down?), a prototype program for water color, in the hope to have this in the gimp.
DigiPencil Realistic color pencil simulation - started from Wetdream, now part of a gimp plugin: PhymodMedia. Nice if you want a more natural media, but doesn't seem to have many options to play around with.
FlexlayPaint. An interesting work-in-progress painting program with supports infinit zoom. It is part of a 2D game development suite, with tablet support, but I haven't tried it.
Those projects I was not aware of when I started:
Krita (KDE, C++) is aiming for the same goal, has an impressive number of contributors listed, already seems to support watercolor and many image depths. I have not yet found configurable brushes, but I haven't tried long because the program is unbearably slow on my PC for even the simplest tiny strokes. (Update: the speed problem is gone, and it even allows custom pressure curves now.)
Gogh (pure python + pygtk) is relatively new, and already supports layers, different blending modes and undo. I expected this approach to be very slow, but it really is not.
And finally mypaint,
my own prototype program which has grown up quite a bit now.