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.
science fiction
Railgun: 2000 Science Fiction Rules
Railgun 2000 is a set of free wargames rules for science fiction armored combat. You need the Fistful of TOWs rules to play.
Savage Survive Skirmish Rules
Savage Survive Skirmish is a set of free wargames rules for games set in a post apocalyptic winter.
Science Fiction Pulp Magazines and Comics
Comic Books Plus has a collection of free pulp era Science Fiction magazines and comics.