The Wrath of Zombies blog has a set of free RPG rules for Mass Combat. While written for the author’s own Thundarr style game, they are applicable to a wide variety of rules and settings.
Thundarr
Under The Broken Moon Role Playing Game
Under the Broken Moon is an RPG based on the Thundarr the Barbarian cartoon series. The game uses a mechanic where you roll a number of dice equal to your trait score and sum the results.