Gary’s Combat Vehicle Reference Guide is what it says it is.
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 New York Public library has put online its collection of color military plates. Most are hand colored block prints. The collection of Mexican Uniforms 1826 – 1906 could come in handy as a painting guide for the Texas Rebellion and the US – Mexican War, as well as the Maximilian period.
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.
The Jackson Gamers have a very detailed guide to gaming the Russian Civil War.