Edgar Rice Burroughs’ Barsoom novels are among my favorite guilty pleasures. Sean K. Reynolds has a page with material, including monster and character profiles from his d20 Barsoon campaign.
The Men and Supermen Role Playing Game is designed to allow super powered characters to fight along side normal characters. In doing this the RPG takes on what I think is one of the most difficult of tasks — creating a common scale between units of disparate strength.
Pulp Avengers is a sourcebook for role playing in the 1930s and 1940s. It’s perfect for providing a realistic background for your games of the Pulp era.
Fickle RPG is
a narrative roleplaying and cooperative storytelling game, distinguished by the core dice resolution mechanic which has bluffing, bidding, and meaningful decisions. The game is a universal system that can be played in any genre, such as sci-fi, military, zombies, high fantasy, superheroes, etc. An ongoing campaign can be played over multiple game sessions, or as a themed one-shot.
Searchers of the Unknown is a set of free role playing rules. These are one-page, minimalist rules, each of which is designed as a stand-alone product. The collection includes: Searchers of the Unkown, Dwarven Glory (and supplement), Karry on Krawling, Witches of N’kai, Adventurers!, Cyborg Samurai Are Go!, Dirty Rotten Scoundrels, Funkin Fists of Fu, Jung Guns, Raiders of the Unknown, The Bastards, Brickmasters of the Unknown, Mutant Got Gunz, Mutant Scavengers of the Ruined Earth (and supplement), Tempora Mutantur (and supplements), Searchers in Spaaaace!!!, Scavengers & Spacewrecks, Swashbucklers of Mars (and a supplement), and Song of the Symbi.