Understatement: Joshua Davis creates some really incredible artwork in Flash.
By combining vector primitives, a carefully chosen color pallete, some deterministic rules, and pinch of random(), the images are generated in Actionscript at runtime. Each time the software is executed, a unique image is created, though it maintains the same guiding principles of other iterations of the same series.
You can download the source for a number of very discrete examples. Tweak and experiment with his code. It’s a nice set of building blocks for creating your own dynamic artwork.