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.
Here’s a large selection of color plates of the uniforms of Norway and Sweden from 1500 – 1909.
The Battle of Marigano is a free board game about a 1515 battle between the French and Swiss. To print it, downloaad the two image files in the middle of the page.
For a terrific collection of color plates of French military costume from the Gauls to 1806, visit The New York Public Library digital galleries.
The New York Public Library Images collection has an amazing collection of the uniforms of Russian armed forces from 960 – 1900. Most are in color. All are useful.