Once again like to point all artists on my feed to this excellent painting app. Been using it over the last month or so and the features its got over other applications stun me, at least for my uses.
I'm not even much of a artist and use it mostly for technical work.
Features such as non destructive transform layers ("transform masks") appeal to me as they have direct analogy in 3d software.
Paint on one layer, have clones/transforms elsewhere that update in realtime to reflect it- its really quite revolutionary for 2d software and you find more use's for it the more you use it.
Other features such as the ability to be able to paint in High Dynamic Range and change the exposure at will, or to have a "wrap around" mode to help make textures are really good for 3D artists.
I have linked to their Kickstarter as they very much deserve a bump, but its already fully free to download with everything I have listed and a lot more.