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!
In Darkest Warrens is billed as a minimalist RPG. Trollish Delver Games writes:
IN DARKEST WARRENS is a minimalist fantasy roleplaying game whose rules cover two pages, including a bestiary and introductory adventure. All you need is some six-sided dice, paper, pencils and, of course, a bunch of buddies to play with.
So what do you get?
All the rules you need, including character advancement for campaign play
Five classes: warrior, rogue, barbarian, mage and ranger
A bestiary containing 23 fiendish monsters to throw at your players
Super easy resolution system – all you need is a single d6
A character sheet
An introductory adventure: The Cow of Boburn
Hours of enjoyment in a small package
Print the rules right out and use them for reference at the table. Perfect for a pick-up and play game.
The game is available as a “pay what you want” from Drive Through RPG.
Weapons of Body and Soul is a martial arts RPG currently in playtesting. The author writes:
The focus of the game is on being a Martial Artist in a land of mythology, where demons and beasts reign and humans can manifest the energy of their spirit to defend themselves. While the primary focus of the game is on combat, the end goal is not about gaining loot. It is about mastering your skills and learning to overcome challenges, becoming greater than you were and achieving your goals.
While it is a combat focused game, you are not required to play a fighter. There are rules in place to play other kinds of characters. This includes a diplomat, or a skill specialist such as an engineer, an inventor, or a smith. WBS emphasises three main design goals. 1) Realism and accuracy. 2) Skill focused gameplay, with high skill trumping high attributes. 3) Tactics in combat, including reasons to use weaker attacks, or to fight at less than your full capacity