Hexblitz is a set of free wargames rules for modern-ish wargames on a hex grid.
Army Dudes is a set of free wargames rules for playing games with the classic green army men.
Snapshot is a set of free wargames rules for playing modern submarine games using 1/700 ships.
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.
SNAFU is a set of free wargames rules for Modern and Science Fiction games. Author Michael Taylor writes:
As for a basic run down of the rules, SNAFU is aimed at 15mm games of about platoon level (each side controls a platoon sized force plus support units) from the modern era into science fiction. The basic unit is the fireteam, usually 4 figures strong but sometimes less or more. As commander of this force you are concerned with the performance of your fireteams as whole units, not of the individual members of each team (leave that work to your team leaders!). As such, units move and fight as one whole.
A key part of the game is stress. The battlefield is a frightening and chaotic place, and how your troops deal with the stress of combat will effect their performance. As units take fire they will accumulate stress points, which makes them more difficult to activate. Adding to the chaos is the random draw deck to determine the order of play each turn. You can never be sure how many units you will get to activate, or when you will activate them.