Skill System
Skills: The Middle Road
Tweaking the Middle Road

Ability Score Training
Autarch Forum

Thief and Fighter Classes, Weapon Damage
Akratic Wizardry

Spell Research and Spell System, Save or Die at 0hp
Necropraxis and his book “Wonder and Wickedness”

Cleric Prayers and Illusionist Powers
9 and 30 Kingdoms

Luck Score
Dungeon Crawl Classics

Encumbrance System
Goblin Punch

Machinist and Bard Classes
ACKS

Solo and Duo PC Rules
Scarlet Heroes

Single Saving Throw
Swords and Wizardry

Naval and Space Rules
Blog of Holding – Naval Combat
Blog of Holding – D&D In Space

Skysailing
Dark Dungeons

Legendary Weapons
9 and Thirty Kingdoms