Necropraxis' House Rules are short and sweet. PDF link.
What Would Conan Do?
- Everyone is an Adventurer
- How to become a God Botherer
- From Unofficial Games: Spells Change Alignment
- Differentiating Weapons
- Breaking Weapons
I like the idea of everyone starting as the same class a lot - I feel like it'd be easier for pick-up games with people who haven't played before. In-game decisions leading to you getting bonus abilities is also an awesome feature of the 'God Botherer' link.
Ten Foot Polemic Unified House Rules and Related Thoughts - very big set of things. I definitely like the first few pages; think I start to shy away in the Necromancer / Barbarian class page. Love the Goblin. Honestly demihumans are a bit played out, from my perspective, but I'd love to see a Fighter MU Specialist Goblin set. I also really like Not Getting Hit Points and general thoughts on Death and Dying.
Games With Others' Extensive List - I especially like the Dark Souls stuff, memory as a disease kind of explains a character having the player's metagame knowledge. The HP setup is also interesting, as are the rules for benching PCs and having backup PCs.
Metal Vs Skin's Rules on classes, race, multiclassing, and goblinoid collective action - I really love the Kalak-Nur setting, and, surprise, I like this post on the classes of Kalak-Nur. This is another LotFP set of hacks. I like how it lets us leave race-as-class alone or let players have their race-class and... adopted? class, via the magic of multiclassing.
Actually, all of Metal Vs Skin's House Rules are pretty great, particularly the HP as countdown clocks. That seems like a particularly hardcore DnD modification.
Lost Pages' B/X Hack: MAGEBLADE. I might throw in the LotFP Specialist and the 'all of you can do this, the specialist can get better' rules, but this is also good as is, and the 5MORE system is easy to comprehend.
The mass combat and horse-riding rules in A Red and Pleasant Land are lots of fun.
Amazingly detailed rules hack just for climbing.
PDF of Telecanter's Receding Rules' House Rules - combat maneuvers are especially well done. I love combat maneuvers like that - like the Mighty Deeds from DCC, simplified, as they can encourage creativity. Having a list of feats is not as much fun, and having no combat maneuvers is even less fun. Zak S put out the monster's guide to combat maneuvers.
If you had already decided on an OSR retroclone of choice, and didn't have enough choice paralysis... Rules hacks! Forever! I'd probably just wind up using Telecanter's or Ten Foot Polemics. I'd like to playtest MvsS's hp countdown, if I ever (ever) have free time again. I want to take some people who want to play an in-person game and run with the What Would Conan Do? ruleset above, set in Vornheim or the like.