The Evolution of Napoleonic Uniforms website is an ambitious work. It is not by any means complete (could any project of this scope ever be complete, given the number of variations?), but is worth browsing through.
The Princely States Report website has some neat color plates of Indian Army unforms from the 1930s and 1940s. They’re originally from a pack of 50 Players Cigarette Cards.
There was a discussion over at the Miniatures Page about a good WWII painting resouce. I don’t normally do posts about commercial products, but you can consider this a public service review.
For my money, the best resource for WWII uniforms is the Armed Forces of WWII by Andrew Mollo.
I’ve painted a lot of WW2 figures on commission over the last several years, and I have found this book to be the one indispensable resouce. Virtually every uniform I’ve wanted has been in here. Where else would you find the uniforms and insignia of the WWII Hungarian Air Force. There are 365 color paintings in the volume, and many more black and white photos. There is also a lot of information about the organization and ranks of the various armies. All, in all, its highly recommended for WWII painters and gamers.
For you colonial gamers, here is a page with useful information on British and Commonwealth Orders and Decorations.
Newcomers to wargaming the Napoleonic Wars can be bewildered by the uniform terminology. That’s where the Napoleonic Uniform Glossary comes in handy. This site has photos of various uniforms and identifies the parts.