Red Hordes is a set of free wargames rules for the Russian Vivil War. They’re similar to DBA/DBM.
Rules: 20th Century
Crimson Eagles Pulp Air Combat Rules
Crimson Eagles is Michael Reitz’s mash-up between the old FASA Crimson Skies pulp aircraft rules and the Canvas Eagles WWI rules.
Schwerpunkt Rules for Warfare in the Age of the Rifle
Schwerpunkt is a set of free wargames rules for conflicts from the late 19th through the 20th centuries.. The author writes:
Schwerpunkt is a set of rules I designed myself before I bought CLA. Basically I asked myself what I wanted in a wargame and came up with the following criteria:
1) It had to be fast Playing
2) It should be capable of producing historical results
3) It had to be fun
With this in mind, I set about using all the rulesets I have ever played or read to come up with what I liked and didn’t like.
Iron Ships and Wooden Heads
Iron Ships and Wooden Heads are a set of free wargames rules for naval gaming “in the era of the big gun.”
Jack Scruby’s Basic Wargaming Rules
Here are a set of rules from the man who is arguably the “Father” of American Wargaming, Jack Scruby: Basic Wargaming Rules.