MyPaint is a very powerful opensource software for artists. It is mainly oriented towards digital painting rather than vector graphics. I used it with my wacom bamboo for this portrait of Antony Hopkins and I really liked to make fun of the thousand tools that simulate pictorial effects. There is a panel also dedicated to freehand drawing. Biro, graphites, pencils, watercolor pencils, wet pen. There really is everything.
Also you can paint on more "sheets" called also here Levels (as in Photoshop) and from the settings panel you can enlarge the thickness of the instrument, make it more or less transparent and you can change it on many other parameters.
This is the result portrait.