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.
20th Century
Life Issues Online 1936 – 1972
Google has the full text of Life magazine issues from 1936 – 1972. An incredible resource.
WRG Infantry Action 1925 – 1975 Rules
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.