Unified Theory Game offers Vortex, a set of free wargames rules for a wide variety of periods. They write:

Vortex is an omni-genre customizable miniatures game for two or more players as infinite as the players’ imaginations. Not only is it possible for a Roman centurion, an old west cowboy, a space pirate and a giant cybernetic alligator to meet on the battlefield, they could find themselves on the same team.

Vortex is a fast-paced game that keeps players involved at all times. The rules are easy and quick to learn while still containing vast amounts of diversity to provide for continually interesting play, game after game. A full campaign system is available which makes each game count, and allows the miniatures to change and progress. The players have the power to write their own story in the Vortex universe. A detailed tournament system is also available for those with a taste for short-term competition.