For many years, George’s Nafziger’s collection of orders of battle for conflicts dating from the 1600s to the 1900s have been a mainstay of the historical wargaming community.
Once available only commercially, Nafziger recently donated his collection to the Army’s Combined Arms Research Library.
The actual files are available as pdfs here. But as they’re numbered and not named, you’ll also want to consult the finding aid here. And a search function.
Here’s one for Flames of War players who may want to pay closer attention to historical accuracy. Bill Stone offers a database of armies of World War II:
Armies of the Second World War” is an online database of day-by-day orders of battle and information about hundreds of division, brigade, and regiment-sized units in World War II. Information currently available in the database covers Commonwealth, Dominion, Colonial, Exile, and “Minor” Allied armies in Europe, Africa, and western Asia from 1 September 1939 through 7 May 1945.