Cthulhu Dark is Graham Walmsley’s free, fast-playing, two-page tabletop RPG about investigating the Cthulhu Mythos and going insane.
Beyond the core rules he’s added basic principles for running it (“focus on the characters,” “respond with fuckery”), detailed guidelines for running it, and tips on scenario creation. And a bunch of optional rules and suggestions for getting more detailed with injuries if you’re into that sort of thing.
He’s still developing it, so go participate because it looks fun.