Drips is a Flash-based painting app that lets you pretend you're Jackson Pollock
Its functionality is very basic -- you splash random colors on the canvas and that's about it. You can select a specific color or allow Drips to randomly rotate colors every time you let go of the mouse. You can also change the background color (again, either randomly or by selection).
There's no undo, of course, but I take that to be in the spirit of Pollock himself. One feature that is missing is control over brush size. I'm pretty sure that Pollock would have approved of such a feature.
I don't think it's a tool for creating masterpieces, but I like its lack of features. It captures some of the raw fun of just painting (or doodling), without messing about with too many features, brushes, toolbars, etc. I also like the strong, bold, random colors.