Google has the full text of Life magazine issues from 1936 – 1972. An incredible resource.
The classic WRG Infantry Action 1925 – 1975 rules are available free for the downloading. These are the June, 1972 edition of the rules. The link opens a pdf.
Empress Miniatures offers ‘And Was Jerusalem Builded Here?’ British Civil War 1938, a set of free wargames rules for playing out the fictional 1938 conflict. From the author:
The following rules are written specifically with the imaginary British Civil War of the 1930’s in mind. They are designed in what nowadays is often called a retro style and pay homage to the style of rules written by such wargaming legends as Charles Grant and Don Featherstone. These rules are designed to give a fun game and so the retro style has had added a very large measure of tongue in cheek humour and period based mischief which hopefully, you are the judge, adds to style of this fantasy period. As such they are seen as being able to be added to by the player. One of the really exciting parts of this period is the way the players imagination can run wild. Therefore many of the sections that follows are guides rather than hard and fast rules. For example unit organisations are pretty fluid and can be tinkered with to suit the players wishes or a specific scenario.
Here’s a collection of color photos from around the world taken by French banker Albert Khan as he toured the world — in 1909!
“When Technology Meets Tradition” is a set of rules for wargaming early modern warfare conflicts, roughly from 1895-1915. The emphasis of the author is on early World War I (1914-1915) but other period wars are also of interest, such as the Russo-Japanese War or the Boer wars. The rules are fast-play and focus on the problems faced by military tacticians during early modern warfare: when outdated linear tactics and love of the offensive ran head-on into lethal, accurate, and rapid firing weaponry.