If you're into digital painting, chances are you know Krita, and if you don't... no, you must know ;P Krita is one of the best apps for painting with your pen tablet, and it's Open Source! I was very excited by MyPaint a few years ago (and I still love it, it's super cool for sketching in an infinite canvas). I liked the brushes behavior a lot, and it was light and very responsive. However, it was lacking some features that I needed very often: selections, transforms, color corrections... so I had to depend on other image editing tools like Gimp and Photoshop to finish pieces I started using MyPaint.
But later, Krita came into my life (it existed before but I didn't know it) with the looks of a tool I could use start to finish for my paintings or texturing works, and I was very frustrated: it looked amazing, included MyPaint's brushes engine, but... it was not available on Windows :P It is available and stable since a while ago, though, and I started to use it more and more often.
Krita is a great tool for digital painting, but there is a lot of room for improvement. And here's the thing. They're running a Kickstarter to gather funds and improve several areas like vectors, text editing, and animation tools (cool!).
So... my recomendation is the following: if you don't know Krita, check it out. Like NOW; you can download it for free at krita.org. And if you know it and like it (which should happen after you try it if you didn't know it before :P), go to that Kicktarter page and contribute with what you can. Even if it is a cup of coffee.