MyPaint is a free open source paint program along the lines of others such as Krita (which I briefly reviewed late last year). The main thing I discovered upon firing it up is that it seeks to be very minimal, with the tools kept at bay as much as possible. I've only spent about 20 minutes with it but it is very easy to get into and I'm having fun with it.
I'm using the Bamboo tablet with this since I haven't installed it on the Surface Pro yet, I have no idea how it will perform on that device. My first impression is that the brushes are cool, being nice and responsive to strokes, and actually, very customizable, so there's a lot to investigate.
Below is the very first thing I did on it with a wacky wet brush that I chose at random.
One thing different about MyPaint is that it doesn't start out with a document size, you just have this (supposedly) endless canvas that you can later crop to the size of the artwork. The aim of this program is to concentrate on creating artwork, it has no manipulation tools, filters or color grading options, and that's fine because it keeps the whole thing stripped back and occupies a small space on your hard drive.