I use Mac paintbrush, which has no layers, really basic tools, a color wheel, a couple of sliders, and custom color spots. That's just about it. I use GIFfun for animations so I basically drag every frame in by hand, then set every single one's frame rate, send it to firefox, and then drag it back to my folder/desktop.
GIFfun is pretty annoying because when I try to put in a folder it messes up the order (which I have tried to fix) and it supports transparency (or whatever). Although, in transparency it can only use black or white as the color, and won't let me do transparent Anti Aliasing.
Paintbrush is pretty fun to use, but it's not nearly as advanced as anything but MS paint.