Every 10 feet fallen beyond the first 10 feet incurs 1d6 damage. If falling more than 40 feet, check CON to avoid death. Hanging from a ledge reduces the calculated fall distance by 10 feet. Intentionally falling causes a Dexterity check; success reduces the calculated fall distance by 10 feet.
Designer's Note: Climbing isn't the easiest thing to make a ruling on, and this system is simple with interesting results.