Microlite 20 is a set of minimalist RPG rules designed for compatibility with Old School Dungeons and Dragons materials, as well as the d20 OGL. Fourteen free Microlite 20 variants are available at the link.
RPG Fantasy
Aketon One Page RPG Based On Chainmail
Aketon is a set of one page (two sided) rulesbased on the classic proto RPG “Chainmail”
Dungeon Geomorphs
The Character For Every Game Blog offers some really neat dungeon geomorph tiles, as well as a few nice complete dungeons. Plenty here to look at.
Open Adventure Old School Adventure Game
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!
Morgalad Fantasy RPG Starterbook
Morgalad is a fantasy RPG set in a pre-medieval world. You can download the starter set for free.