I made a little Java drawing experiment called Tendron. It starts with a few lines and turns into a branching, rather plant-like scribble. There is not really a game here; I just wanted to see what the pattern would do.
A Cluster creates four Tendril objects pointing into different quadrants. Each tendril draws short randomized line segments, then creates another cluster at its endpoint with a shorter length. The recursion stops when the length gets too small.
That is the whole trick: a simple shape calls a smaller version of the same process, and the picture gets complicated on its own. The drawing changes when it is painted again because the step sizes are random. I like how much can come out of a small recursive rule without having to describe the final picture ahead of time.