Most veteran readers will be aware of some or all of these, so I am treating this post as something of a quick-start guide for people new to the DIY sphere who want some good places to start customizing.
Shields Shall be Splintered
The classic: sacrifice your shield to negate damage from an attack.
While the example is for sci-fi there's no reason this can't be applied wherever for many more varied and special weapons and armor.
Less of a house rule and more of just a class feature that adapts firearm rules to other powerful weapons, but I really like the mechanic: Ignore AC from armor and apply DR instead, roll 1 size higher damage die if you are using two hands.
Instead of just dropping dead, you can get a fancy scar and lingering trauma! It's fun - players can survive a bit more, but there's always consequence and choices to follow - keep going, or retire? See if it can heal, or make a new character? Having it split up by damage type is also great fun.
Heavy vs Fast Weapons
It just makes sense, you know? Use STR for bows and weapons of d8 damage or higher, and DEX for d6 or lower and other ranged weapons. Simple, easy, sensible.
DCC's luck stat is so good it should be used everywhere. It's a tool to solve problems on the fly, a means for players to have additional input on goings, and a resource to manage and make decisions with. 3d6 as normal, burn a point to gain +1 on a roll, roll under for a check. That's it.
Vaginas are Magic Casting
You can safely cast a number of spells equal to your level per day. Any further, and you have to make a save vs magic or roll on a terrible magical feedback table. (See AuraTwilight's wild magic for ultra funtimes)
You have slots = to your CON. No fuss. Heavier armor and weapons take up additional slots.
Extra Stuff for StartersDarkest Dungeon Character Kits
I will recommend this to everyone who will listen and quite a few people who won't. It's the single best starting point for pre-made characters that really get the old school motif, and a great way to demonstrate to new players that classes are not monolithic: not all fighters are the same.
Is fucking awesome, is what it is. If you need a hub city, this is your hub city. Modify to taste. Use it for Sigil, for Ravnica, for Throne, the Yellow City, whatever.
The Infinigrad Tables
The best way to populate Corpathium.The job tables in Blasphemous Roster have saved my bacon before and they certainly will in the future.
Every god damn Trilemma adventure
They're short, they're free, they're great.
100 Minor Magical Items and 100 Problem-Solvers
Gives treasure some personality and use.
Other Stuff"The Coolest Thing Ray Ever Did" by Jeff Rients
"Orcs, Violence, and Evil" by Emmy Allen.
Ramanan's automatic level 1 character generator
Purple Sorcerer's level 0 party generator.
Links to Wisdom and the OSR Planet
Benton Molina's Incunabuli is some bloody fantastic prose and I can't wait for the book.
Signs in the Wilderness is mythic Americana and I can't wait for the book.
There's more, there's always more, but these are the ones first in line.