FORGE is a simple, quick to learn roleplaying game with a significant amount of GM tools thrown in. It's earlier version was based on Knave by Ben Milton, however it has since been overhauled and massively expanded, whilst remaining fully compatible with all OSR and B/X D&D resources.
The 60-page FORGE book is divided into roughly two halves:
Core Game System (d20):
Streamlined rules, originally based on Knave by Ben Milton
Slot-based inventory system
QuickStart character equipment bundles
Abstract distances (Close/Near/Far etc)
Rules for Hex-crawling and Dungeon-delving
Overhauled magic system
Downtime activities
Expanded follower mechanics
Domain rules
Quick References on a two-page spread
Hand drawn character sheets and worksheets
GM and Solo Tools (d6, 2d6 and d66):
d6 Oracle with variable likelihood, plus Éand/Ébut modifier
Random Event Generator, tied in to the Oracle
A set of procedures outlining how to incorporate the GM Tools with the core game system
Quest Generator
Wilderness Generator
Dungeon Generator
Settlement Generator
Creature Generator. Plus a two-page spread of Example Creatures
Character Generator
Name Generators
Treasure Generator. Plus a page of Example Magic Items that interface with the core game system.