TeraFractal

TeraFractal is a free Mac OS X application that lets you create fractal images by dragging cute little boxes around.
Mathematically speaking, each box represents a transformation in an iterated function system -- but you don't need to understand fractal math to have fun with TeraFractal.

You simply use the mouse or arrow keys to alter the position, rotation, and size of each box. TeraFractal updates the fractal image continually as you drag the boxes. This real-time feedback makes it possible to create lifelike fractals in a surprisingly short time.
Be sure to also check out the new multi-touch version for iPad, iPhone, and iPod Touch: http://www.terafractal.com/ios/