I like simple, and sometimes there is such a thing as having too many bells and whistles when all you want is pure simple. Gogh does this for me. Gogh is a ridiculously simple, yet beautiful drawing/sketching program that just gives you the basics: pen, pencil, smudge, colour and an eraser. It works great with Ubuntu 10.04, and is quite stable.
There is a bit of a trick to starting after you uncompress it – you must either start it up in terminal (the Gogh executable file),e.g., “./gogh,” or simply right click on it and choose run, or create a short-cut, but the folder must stay somewhere in your home directory. You can download it from here: GOGH.
Both of these sketches took only seconds to make. They are not perfect, but I like the feel of the program, so any mistake you can either erase yourself out of trouble, or hit the undo.