DiceDream

There is now a 2nd edition: https://taylor-lane.itch.io/dicedream-2e

This is a dungeon-crawling game, pure and simple. It takes influence from a number of other such games, and is designed to be compatible with any content with the same six stats, levels, and Hit Dice as Dungeons & Dragons.

It incorporates Dis/Advantage from 5e D&D, 3d6-straight-down stat generation from old-school D&D, Aspects and Fate points from Fate, trianary task resolution from Apocalypse World, and incorporates some of my past work. 

The game’s mechanics are hyper-focused on dungeons, to the point that the mechanics for time-tracking and even for it’s equivalent of HP stop applying when in the surface world. It’s meant for the GM to choose a megadungeon, and have the PCs spend all or nearly all of their time either in that dungeon or in the closest settlement, preparing to go back into that dungeon. It’s great in that context, terrible in any other that I can imagine. 

That's all just a direct quote from the introduction. It's a 10 page minimalist OSR game with storygame influences. It's backwards compatible. It's free.