Apropos of nothing... we've made Basic Roleplaying aka 'The Big Gold Book' only 99 cents on DriveThruRPG (normally $21.95)*.
At some point, almost every gamemaster wants to create their own Mythic World. That's what the Basic Roleplaying System (BRP) is all about! It is simple, fast, and elegant, using a skill-based percentile system. Combat is fast and deadly, and with a strong focus on non-combat skills that some game systems are challenged to support. Players roll percentile dice against the Gamemaster-led opponents, or even themselves.
At the core, most Chaosium RolePlaying games use a variation of the Basic RolePlaying System, which started with RuneQuest. Call of Cthulhu, StormBringer, SuperWorld, Pendragon, and others (including, most recently Rivers of London) followed.
So, if you want to create your own Mythic World and leverage a proven system that is the backbone of some of the most successful games in RolePlaying history, look no further!
Also check out the BRP Wiki.
*We'll keep it that price until the end of DriveThruRPG's current 'New Year - New Game' sale (ends January 19th).