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.
The Krigsspll.dk site offers a very large selection of downloadable flags for the Crusades. They are all very well done.
The New York Public Library’s digital holdings have an amazing collection of color plates of British uniforms from 1066 – 1900. These are great painting guides.
Merlin Larson offers a list of battles with Vikings from the Anglo Saxon Chronicles. The most interesting thing he found was that – contrary to the mythology — the Vikings were not necessarily superior in battle to the Anglo Saxons.
The New York Public Library Systems Digital Library has digitized images of thousands of plates, prints and paintings, including these of the uniforms of Byzantium through 1453.