Quick Quest: Basic RPG

Grab a single d6, some scratch paper, pencil and head to the dungeons. This absolutely free, retro-style little roleplaying game has everything you need to get started on a fantasy adventure. Included: rules for character creation, sample spells, combat and task resolution, along with quick guidelines for creating a bunch of no-frills monsters in a hurry. 

Is it perfect? No, not by any means. But it's yours for the low, low cost of nothing, and it'll be a fine base for home brewing anything you want.