The British Museum has a very nice presentation on the art, culture and …. warfare! … of ancient Mesopotamia.
The Internet Modern History Sourcebook is an amazing resource for all students of history — which includes, I’d guess, most historical wargamers. I’m constantly amazed at what is available on the internet today. Materials like this would take up many, many square feet of shelfspace in my little study.
The Oliver Cromwell Society has a website with basic information on the English Civil War. In light of the growing popularity of Warhammer Historicals’ English Civil War miniature wargaming rules set, this could be useful in filling in some gaps players’ knowledge.