Here are a couple of things that I've come across that some might find useful.
The first and the best is Alchemy. It creates shapes, lines and special effects in a semi-random manner that is great for generating ideas. You can also produce great drawings. http://al.chemy.org/
Then there is ColorJack, a great color theory and palette resource. http://www.colorjack.com/
And it has a nice online sketchpad partner. http://mugtug.com/sketchpad/