Flocking (running a large number of independent agents with simple proximity-based rules and letting interesting behavior emerge) has been a popular graphics technique since the 1987 SIGGRAPH paper by Craig Reynolds.  The idea is, of course, inspired by examples from the animal kingdom such as bird flocks and fish schools.  Today I saw an internet clip of 300,000 (!) starlings flocking. With such a large number of entities, the flock looks like some kind of bizarre physical fluid or smoke simulation.