Even in a fantasy world, apocalypses happen. There are those that survive them simply through grit and determination. And when they get back to society, they're sought out for their strength and toughness. Skirmisher Publishing has released rules for these post-apocalyptic barbarians. The book's primarily for Pathfinder, but can be altered to fit other D20/OGL rules sets.
From the website:
The wasteland is a cruel place, coddling no weakness and rewarding only strength and tenacity. Out of its blasted terrain ride a people as harsh and deadly as the irradiated land that bore them. Survivors, scavengers, and throwbacks, they are all that remains when warfare and catastrophe burns away the façade of civilization. They are Post-Apocalyptic Barbarians. Born from the ashes of world-shattering conflicts, they are conflict incarnate. Driven by rage, they live by blade, bullet, and wit. Making use of whatever tools and technology they can cobble together from scraps of metal and rusted machinery, they lurk within the husks of crumbling factories and ride on tides of screaming steel.
This basic character class is stat'ed for the Pathfinder RPG but can easily be used in conjunction with any OGL/d20 system games.