Inkscape. Inkscape is a free program that is used for drawing/editing. What is cool about it is that it uses vectors to map everything you draw or import so you can really play around with every stroke you make. There are pencil and calligraphy tools and so many fun things to play around with. And as everything that has to do with Linux, it is absolutely free! (and you can use it on Macs and Windows, too).
So here is a drawing I did, mostly from memory, playing around with the calligraphy tool.