Battle of la Drang is a set of free wargames rules for battles between the US 7th Cavalry and Vietcong forces. The battle of the title was the basis for the book and movie, "We Were Soldiers."
Double Tap is a set of free wargames rules for games involving special operations forces in the near future. The rules are just one page, but look as though they are anything but simplistic, as they use a reaction system.
Grunts, Leathernecks and Charlie is a set of free wargames rules set in the Vietnam War. The author writes:
Players who are interested in a FUN and FAST Vietnam game system can consider these rules a flexible framework for use. I designed these rules to be used by both beginners and veteran players. Play can be quick and easy OR made more realistic and slower by adding advanced options. If you do not trust your opponents, then this is not the right rules’ set to use. It will assist any player who has an interesting scenario to run at a convention or at a local shop. The Combat systems reflect volume of fire into a specific area combined with the maneuvers of combat units.