Game Engines

FuRPiG is a full-blown simulationist rule set, with accompanying skill-sets for several fantasy and SF backgrounds, that I developed and ran with Jim Burrows and Tuesday/Friday night group for years and years.
Harry Potter and the Enchanted FuRPiG gives a short add-on for FuRPiG, for gaming J. K. Rowling's famous setting.
Endeavor is a light mechanics system, using the percentile dice and learning rolls from FuRPiG, the trait system from Over the Edge, and a "try harder" system of resource points.
Larger Than Life is an add-on for Endeavor, for "surreal" role-playing involving changable levels of reality and a miracle system.
Make Three is a light, diceless mechanics system for supporting narrativist, story-telling-style role-play. It uses elements from Over the Edge and Amber Diceless.
Noodle Incidents are a humorous way of filling in the gaps when you want to fast-forward through a long time in a role-playing campaign, designed for Lords of Being.
Backstory Buttons are a generalization of Noodle Incidents.