Battle For 2mm Earth is a set of free wargames rules for pre WWI wargames in 2mm scale.
The Battle Game of the Month Blog has the outline of a set of free wargames rules for playing games set in the Russian Civil War. They’re inspired by Joseph Morschauser’s rules.
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.