Krita is a really awesome open source digital art program. While many graphics programs focus on editing and design features, Krita is really designed for drawing, sketching, painting, etc. It’s brush system is quite extensible and is capable of recreating a variety of different media types.
Krita is best used with a graphics tablet, like those from Wacom (which seems to be the standard in such things) or others (which might be a lot cheaper), but using it with a mouse is doable for some things.
Resources
visual keyboard shortcut cheatsheet