A fractal growth model that approximates lightning-like effects (like Lichtenberg figures), with java-based implementation:
http://apricot.ap.polyu.edu.hk/~lam/dla/dla.html
"The growth rule is remarkably simple. We start with an immobile seed on the plane. A walker is then launched from a random position far away and is allowed to diffuse. If it touches the seed, it is immobilized instantly and becomes part of the aggregate. We then launch similar walkers one-by-one and each of them stops upon hitting the cluster. After launching a few hundred particles, a cluster with intricate branch structures results."
