deremilitary.org is an outstanding website. It's great because it is maintained by academics and holds to scholarly principles. It's also reasonably well organized.
The site, as the name says, is about military history. What the name says only by inference is that it is only about pre-modern military history, so it's not burdened down with endless WWII files. For those of us in ancient or medieval history, that's a real boon.
The site has numerous scholarly articles, and even a few books, online. All open to the general public, as *all* scholarship should be. In addition, it has links to radio broadcasts, primary sources, and course syllabi, among other resources. There are special sections for the Crusades, the Byzantines, the Anglo-Normans, and the Vikings. It holds the Journal of Medieval Military History since 2003. The bibliography at the site is extensive, current, and informative. Finally, there's both a blog and a forum. In short, this should be a stop on any researcher's first, second or even third journey into the topic of medieval military history.
The site is also available in English, Spanish, French, and Italian, though the articles are in whatever language in which they were written.