I don't know if you have used the Facebook app, Graffiti, but I quite like it. These days I don't have time to play with it unfortunately, but basically, it's a very simple drawing tool. It distinguishes itself from other drawing tools by not allowing "erase" function. You can only paint over what is already drawn (just like graffiti'ing on a wall with an air brush), although you can "undo" your last action.
Some people are so talented, or simply have too much time on their hands.