Shadowdark is a rules light game system based on old school Dungeons and Dragons. The core concepts should be familiar to anyone who has played modern editions of D&D and can be easily picked up by those who have never played before. Minimal rules are meant to guide the story rather than dictate what can and cannot be done, with many decisions ultimately left to the Game Master to decide. It seeks to instill a sense of great reward for great risk. A free quick start introductory book can be found here.