Langton's ant
It is interesting, but, pardon my ignorance, is it appropriate to call this "a behaviour" (in the traditional meaning of this word)? It looks to me like "just" playing with numbers, i.e. normal phenomenon ultimately describable with some math theory. It looks far-fetched to me to describe it with the terms like "simple rules can result to complex emergent behaviour," etc...I remember there are some number sequences generated by some rules, which seems random at first and then some sub-sequence repeates. Now this looks similar to me, except it seems to more mysterious because someone described it as "ant doing something". ;-)