Krita is worth your consideration. It is a very impressive alternative to Photoshop, Painter, even Manga Studio and Toon Boom Animate (the animation tools are currently being tested in the most recent beta)
I haven't given up on Adobe's old workhorse, but I do actually prefer Krita for painting and drawing. I find it much more responsive and intuitive and I like the brushes more. Plus, it does multiple undos without the need of a clumsy history stack. The animation features are really good, too.
The bonus is that it is open source, multi-platform and freely available, although I would encourage anyone who likes the program to consider some kind of donation in order to keep the developers funded so they can continue to improve it.