The Conquest of Mexico is a period I’ve always wanted to game. Steve’s Balagan website has a very useful web page of information and links to resources on the Aztecs and other inhabitants of Mesoamerica.
The Lord of the Rings Yahoo group has army lists for playing a Aztec v Conquistators variant.
New World DBA is a set of amendments and lists for playing DBA games in the New World: Tupis, Aztecs, Incas and Conquistadors
In the Jungle is an interesting set of rules for conquistador vs aztec games. It’s played on a gridded board and is supposed to simulate a group of “lost” conquistadors trying to find their way to a temple without falling victim to sacrifice.