Grand Internet Sources Thread!

[TinCow]

Here are some of the sources I use occasionally at work. Most span the entire history of US military history, so I'm not quite sure where you would categorize them. Somewhere between Napoleonic and Modern, I would guess.

Dictionary of American Naval Fighting Ships - Histories of every American warship ever built
Marine Corps Historical Publications - All of the official USMC historical books, available online in pdf format. Very, very detailed accounts of USMC actions from Tripoli to the Persian Gulf War. The books are heavily researched and of the highest quality. I highly recommend this site; it is essentially a small library available for free.
National Archives and Records Administration - Archives Database - All kinds of information on American history, but with a very nice section on many, many aspect of the various US wars
NavSource - Photographic History of the US Navy - Numerous photographs of nearly every US Navy and Merchant Marine ship ever built.
Navy Historical Center - The "Online Resources" section has a great deal of information on all aspects of US Naval history.
Vietnam Veterans' Memorial Wall Online - Exactly what it says. Information on every US KIA from the Vietnam War. Many include comments from people who knew them with more specific data.
US Coast Guard Historian's Office - Information on all aspects of USCG history.

Some other personal favorites from my links list:
Axis History Forum - Great for WW2 info of all kinds
Historical Atlas Map Collection - Historical Maps from Ancient to Modern times.
Livius.org - A great Ancient history info website
Roman Empire.net - Lots of Roman info