Free RPGs are more common than ever. Here's a lovely blog post that details how to get your hands on Free Rpgs of all sorts (completely legally). I believe that you'd call them open source, in the parlance of our time.
(Unfortunately, Call of Cthulhu is NOT amongst them.)
Call of Cthulhu Quickstart 7th Ed Rules available here.
However, included in the list is perennial Cthulhu Dice favorite Lamentations of the Fire Princess (or OSRIC, very similar), as well as mega popular games like Pathfinder and new D&D, as well as other crowd favorites like FATE, and Dungeon World.
What a world we live in! It's a good time to be an RPG fan.
RPGs went Sharware
