Threat is a modern micro armor game. The authors write:

THREAT is a modern-era miniatures wargaming ruleset that was designed to be a fast-moving fluid game of modern tactical ground combat using 1/285 scale miniatures.

THREAT is a very realistic simulation of the deadly modern battlefield. Combat is simultaneous, for this reason a referee is recommended to direct resolution of the firing and resolve line of sight (LOS) problems. Each miniature represents a one-for-one ratio of personnel and equipment. Aircraft may represent multiples of aircraft of similar type.

Unlike other game rules, THREAT does not use individual vehicle charts, rather it uses charts for each weapon system that modern vehicles employ. In this way any number of vehicles with different weapons mountings can be utilizied without having to create new charts.

The Die is Cast – Rules For The Battle of Pharsalus (48 BC)

The Die is Cast is a set of free wargames rules For The Battle of Pharsalus (48 BC), a decisive engagement between Caesar and Pompey.

Link

Mike Mike Projectile Wargaming

Mike Mike Projectile Wargaming is a set of free wargames rules for games of that sort where you use some short of shooter (such as a rubber band gun) to knock over the figures.

Use with plastic only.

The Portable Wargame: Ancients

The Portable Wargame: Ancients is a set of free wargames rules for ancients wargames using a gridded surface and individually based figures. The Ancients version is one of several using the same system in different periods.

Wargaming Miscellany: Free Downloadable Wargame Rules

Flip It Paper Combat

From the authors:

Flipit: Paper Combat is a fast-paced tabletop miniatures game in a setting similar to WWII. Players lead forces from one of the four major nations into the conflict for Jurop. Armies are built of models players can download, print and build for almost free (you still have to supply the ink, paper, and glue). There are simple starter rules available, which may be supplimented by optional advanced rules to make gameplay more challenging

Hat Tip to Martyn for the link.