MyPaint is an easy-to-use, open-source digital painting application.
Open the program and you'll see little beyond a blank screen, colour selector and wide range of brushes, then. The latter start with the familiar - "pencil", "pen", "marker", "watercolour", "sponge" - but soon branch out, with realistic oil, pencil or charcoal, and move on to the experimental "fur", "leaves", "DNA-brush" and more.
If that's not enough for you then you can tweak the built-in brush engine to produce new styles of your own.
And support for pressure-sensitive graphics tablets then helps you produce quality artwork with minimal effort.
The program also includes basic layers support, which means once you've created a background then you can begin to paint above it, without affecting once you've done before. And if you make a mistake at some point, of course there's an unlimited Undo option to reverse the most recent changes you've made.
The end result is an interesting and powerful package; if you've the talent, then MyPaint can help to bring it out. And for proof, look no further than the program's Galleries, which have some great examples of artwork produced with MyPaint.