gimp is pretty good if you're more into editing rather then fresh drawing/painting, otherwise you should check out the 'light' version of Paint Tool SAI ( it's called "Easy Paint Tool SAI" as far as I remember <.<)
MyPaint is pretty good. It doesn't have a select tool, but it's got an infinite canvas, can be controlled entirely through hotkeys, a wide array of brush customizability, and some basic layer support. I find it's best for doing character design. Customizing brushes takes forever though - it's a REALLY wide array.