I've decided to trim this down – in 2023; Google will get you a lot of information right away – consider this page a relic of the “First Age” of the internet, where search engines were quite bad and the links page on each site led to interesting places.
Liberated Manuals – Has a significant number of U.S. Military manuals in freely downloadable (PDF) form. Created solely to ‘liberate’ manuals from unscrupulous CD-ROM sites which dominate google rankings. There is a limit of 100 manuals per IP address per day, designed to protect the website, but they offer torrents if you need to get them all at once.
TankNet – Armor Themed Discussion Board.
Secret Projects – Covers a wide variety of “what if” projects from a variety of disciplines (Air, Naval, Ground). Many published authors post there.
Shipbucket – Devoted to drawing pixel art of various military equipment (ships, aircraft, tanks). Be warned though, they exaggerate height by about 20% due to inherent limitations of the scale they work at.
Air Force History Index (Private) – This is a PRIVATELY run website not not affiliated with the United States Air Force or the Air Force Historical Research Agency. What it is is an index of 550,000 documents that the Air Force Historical Research Agency has, so that you can order the digitized microfilm reels from the Air Force rapidly. For example, you find out that documents are on Reel A2011 through the Index and you can order that reel from AFHRA and get it in a week or two; instead of writing them with “Do you have the War Diary for the 404 Fighter Squadron” and waiting months to get a reply back.
Air Force Historical Research Agency (US Air Force) – The USAF's official historical site.
Naval Historical Center (US Navy) – The US Navy's official historical site. Very large and comprehensive, especially their photograph section.
Combined Arms Research Library (US Army) – The US Army's Combined Arms Research Library at Leavenworth. Has a very large and extensive PDF collection of operational documents and historic field manuals.
U.S. Naval Vessel Register (US Navy) – Gives you relevant information on any ship the US Navy has owned since World War II.
National Archive, Kew – The British National Archives. A lot of stuff is now available as downloadable PDFs.
National Archives – The United States' National Archives. Some stuff is now available online; mostly WWII USN war diaries. Their website is mainly to be used as a starting point for your own research on-site at their locations.
Defense Technical Information Center (DTIC) – The US Defense Department's Archive. Contains in it's archives many PDF files relating to various technical aspects of military equipment. Worth a look every once in a while.
DOE OpenNet – Archive of Declassified documents by the Department of Energy, which controls US Nuclear Weapons production.
Civil War Soldiers and Sailors System (CWSS) (U.S. National Park Service) – Very comprehensive list maintained by the NPS of Civil War Era records on Soldiers, Sailors, Regiments, Cemeteries, Prisoners, Medals of Honor, and National Parks.
U.S. Army In World War II Series (The Green Books) – U.S. Army “Green Book” Series, available online in HTML/PDF format.
Library of Congress, Farm Security Administration-Office of War Information Collection – A significant number of the color photographs concern the American mobilization effort for World War II and portray aircraft manufacturing, military training, and the nation's railroads.
University of North Texas Digital Archives – American War Posters – This contains some 380+ US propaganda posters.
Perry-Castañeda Library (PCL) Map Collection – An excellent collection of maps for the entire world at the University of Texas.
Anthony G. Williams' Website – Author of many books on cannons, machineguns, etc; his website is a good supplement to those books. I fear that he has died, since his site is gone. He originally was at http://www.quarry.nildram.co.uk then he moved to http://quarryhs.co.uk. The last time quarryhs.co.uk was up was in December 2022 (Internet Archive Link to 15 Nov 2022 Crawl).
Achtung Panzer! – One of the oldest websites on the internet related to German Panzers. Has been online since December 1996, some 27~ years(!!!)
TANKS! – Armored Warfare Prior to 1946 (Internet Archive Version, c.2005) – This was one of the earliest websites to detail pre-WWII tanks for a lot of minor powers. It's long since been gone (Original Link – Now Dead), but you can recover the 2005 version through the Internet Archive.
Russian Battlefield – A very excellent website on the various technical aspects of Russian military equipment in WWII.
Dictionary of American Naval Fighting Ships – Published in nine volumes (from 1959 to 1991), DANFS gives histories for virtually every US vessel. There are two versions:
Haze Gray's DANFS Version – The unofficial private version that was done collaboratively from 1996-2003.
Naval Historical Center's DANFS Version – This official version was done circa 2004+
Ship Scribe – Stephen S. Robert's website and is a continuation of his work done for Register of Ships of the U.S. Navy 1775-1990: Major Combatants. Major sections are:
Register of Ships of the U.S. Navy: Auxiliary Vessels 1922-1945
Steel Shipbuilding under the U. S. Shipping Board, 1917-1921
French Navy Ships, 1816-1859: From Sail to Steam
https://www.german-navy.de – Devoted to German Naval history and a small amount of Great Naval Battles stuff.
David Dickson's IJN Page – This site has a lot of IJN translated documents, etc.
Haze Gray and Underway – Naval Photography and History Website. Has been around since 1994.
U.S. Maritime Commission: 1936 through 1950 – Shipbuilding under the US Maritime Commission, with a lot of detailed drawings and statistics.
The Coast Defense Study Group (CDSG) – A group devoted to studying US Coastal Defenses. They sell a quite significant quantity of digital copies of original documents from their store.
Hikoki: 1946 – Website covering the experimental aircraft of the IJN and IJA.
http://www.joebaugher.com – Joe Baugher's Home Page – His “American Military Aircraft” section is one of the finest collection of articles on aviation on the internet. Highly recommended.
Luft '46 – Comprehensive website on all the German “Napkinwaffe” Paper Projects during WW2.
Designation Systems.Net – Semi-Comprehensively covers virtually all U.S. Missiles and guided weapons from the 1940s to circa 2018.
Warbirds Resource Group – A very long running (since 1996) website on WWII aircraft, etc.
WW2 Warbirds by Frans Bonné (Internet Archive Link) – A now dead website from the early 2000s which had a lot of information on WWII aircraft. (Original now-dead link).
University of Alabama – Huntsville (Saturn V Collection) – Lots of 1960s era MSFC documents.
US Army Patches – Website detailing just about every Army Patch made, with scans of them.
HyperWar – Website based around gathering as many primary source documents from World War II as possible. Extensive online collection of US Government official histories.
Lemairesoft's WWII Encyclopedia – (Internet Archive Link) – A long dead website that had a lot of interesting stuff. (Original Dead Link)
Da Font – A Freeware Font Website with minimal pop ups and a decently large collection of fonts.
OpenTTD – Open Source remake of Transport Tycoon Deluxe. Highly modifiable with many free mods available.
www.xcomufo.com – One of the internet's main X-COM Pages, with a large amount of files for all three of the main turn-based games in the series (UFO/TFTD/APOC)
Scott T Jones' XCOMUTIL Homepage – This a very useful editor for the original UFO/TFD series. It's now kind of obsolete, with OpenXCOM fixing many of the issues that required it.
Fred Prado's WCSII: TANKS! Page (Internet Archive, May 2017 Link) (Original Link) – Fred Prado's page on Wargame Construction Set II: TANKS! and how to get it running in DOSBOX, files etc.
Sub Sim – Subsim has been around for nearly 25 years now and it's the best site for any submarine-related game, e.g. Silent Hunter III/IV, Dangerous Waters, etc; but the Steam Forums have been poaching readers away from them for a while now.