I recently read a blog post by Richard Byrne on "Seven Registration-Free Drawing Tools for Students" (Free Technology for Teachers). One really cool thing about these drawing tools is that they are all web-based and students do not have to have a log-in to start drawing.
My favorite is Bomomo. When you go to that web site, you have a blank screen with circles moving around. Your mouse controls the circles. At the bottom of the screen are all sorts of patterns that you can use to create art. As you choose different patterns, different colors are chosen also.
Below is my art from Bomomo. I think it turned out pretty well.