Fields of War is a set of free wargames rules for fantasy armies. In addition to the core rules, there also are a large number of interesting army lists.
rules
Snapshot Modern Submarine Rules
Snapshot is a set of free wargames rules for playing modern submarine games using 1/700 ships.
WW1 Air Combat Rules
Don offers an updated version of his air combat rules, which he now calls Aerial Craps.
WWI Fantasy Rules
Orks and Dwarves in a WWI setting? Sure. Here’s a set of fantasy WWI Rules.
The Portable Wargame: Modern Rules
The Portable Wargame is a sort of mini-movement in the community: a series of user created rules for games based on checkers boards. They all look interesting and fun. Here are the rules for the modern version.