TeraFractal is a free Mac OS X application that lets you create complex fractal images by manipulating colored boxes in a control window.

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 (translation), rotation, and size (scaling factor) of each box. TeraFractal updates the fractal image continually as you drag the boxes (using an efficient randomized algorithm). This real-time feedback makes it possible to create lifelike fractals in a surprisingly short time.