Stuart Robertson ventured into related territory for his Weird West Roleplaying Game Basic Rulebook. Available for considerably less than a fistful of dollars, this little (and I do mean little) game distills an entire old school D&D-style rule set onto a single PocketMod.
I just can't stop thinking about this game.
The design choices almost perfectly match what I would do with a D&D game if were to take it out of its natural dungeon fantasy environment. Ability scores are gone and with them any barriers to envisioning your character the way you want. The characteristics of the core D&D classes (fighting, hit dice, magic use, and "skills") replace ability scores and a nice mix of four classes ("Paths") allow these to advance in different ways. Thinking of my own weird west setting, I could imagine converting most of the native characters to this system.
The combat stuff is pretty cool too. The turn sequence has been drastically simplified yet there are seven interesting actions/maneuvers that give combatants meaningful choices. And I love the inclusion of a stunt action as it seems loaded with possibilities. Defense (AC) now tracks with Skill, making unarmored characters viable. That change also corrects a nagging issue I have with old school D&D combat - namely that a character's offensive abilities improve while a character's defensive abilities (outside of hit dice) do not. And speaking of hit dice, I really like how hit points ("stamina") are handled in the game. One of several novel stamina-related rules is that interesting stuff happens to a character when they are reduced to 0 (death being only one possibility). Stamina points are also rerolled every session.
All in all, this is a fine "rulebook" that can be carried in your back pocket or on your phone (if you haven't already memorized it). It's broadly compatible with D&D and my first thought would be to use this in a B/X-style setting with weird science thrown into the mix. It would also be great for Trey Causey's upcoming Weird Adventures (teased on From The Sorcerer's Skull).