Some games are complicated - they have really complex rules. Examples of this are games where you have to track many different types of tokens, with different rules for each.

Other games have really simple rules, but still manage to produce extreme complexity in they way they are played.

Go is the perfect example of this. It literally has 2-3 rules, but because you can play anywhere, the complexity it can produce is wide and deep, to the point that tomes have been written on how to play it.

What other boardgames exist that have very simple rules, but produce complex and interesting game play?