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.
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.
Lizards vs Wizards is an RPG set in the “days after the Big Blammo.” You are a sentient lizard. Unfortunately, there also are Wizards, who are “crazy, weird and dangerous humans” who often use lizard parts in their experiements.
It looks like a hoot.
GDW’s Classic RPG Traveller from the late 1970s is currently available for free at Drive Through RPG.
Dead Simple Steampunk is a set of one-page RPG rules for settings in a Victorian science fiction setting. There’s also a one-page setting and one page for equipment.