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.
It used to be Paint.Net but I don't believe they support tablet pen pressure anymore.
I've really liked MyPaint and occasionally use it to work on sketches/concepts/ideas because it has an infinite canvas. Other than that I don't use any other free programs although I've tried many.
Maybe Gimp will work for you? I've never liked it, but maybe it's better these days, it is the most advanced (more tools).