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.
Fires of Space Rules
Fires of Space is a space combat game with a WWII naval flavor. From the author’s introduction:
Here is my take on battles between big ships in space. This rules set has a deliberately 20th century naval warfare flavor, but with adaptations for space and far future technology. Included are the rules, tables, designer’s notes, a series of ships from various space navies, scenarios, a probability table for 20 sided dice, counters, and a 50 by 50 hex map. With the map are marked map sections showing showing the map’s center hex and deployment areas for tactical scenarios. Ideally, the game is played with two 50 by 50 hex maps: one for operational movement and the other for tactical engagements. In a pinch, a single map can double as both the operational and tactical map. Finally, I include a copy of the Excel workbook that I used to create the ships in the game.
Science Fiction Pulp Magazines and Comics
Comic Books Plus has a collection of free pulp era Science Fiction magazines and comics.