The Miniature Wargaming Editor The Lost Worlds game books are a classic game where two players face off against each other using illustrated booklets that cross index the results of their combat moves. These rules are for using miniatures with the Lost Worlds books.
Quindia Studios has a set of free Frostgrave character sheets, done in an “old school” style.
Frostgrave is a set of rules from Osprey Publishing about wizard-led gangs searching for loot in a frozen city.
Open Adventure is a free, open-source old school role playing game. From the description:
Open Adventure is a full-featured tabletop adventure game that offers out-of-box support for medieval fantasy and science-fiction expeditions. In Open Adventure, you play as a heroic or villainous figure free to explore the perilous world created by the all-mighty Game Master. Along your adventure you’ll battle ferocious enemies, solve complex puzzles, navigate labyrinthine mazes–and much more!