Teratic Tome
Ford's Faeries
Rifts World Book 1 - Vampire Kingdom - Original Printing as it has the stuff for Circus and Traveling Shows
The Original Ultimate Toolbox for GM by Ibach, Ibach, and Pinto
Cites by Midkemia Press
Tome of Adventure Design Revised
The Arduin Trilogy
Hex Flower Cookbook
Kellri's City Block https://kellri.blogspot.com/
Kellri's CDD1-4 https://kellri.blogspot.com/
Justin's House Rules https://thealexandrian.net/wordpress/7842/roleplaying-games/justins-house-rules-for-odd
Gary Gygax's House Rules https://cyclopeatron.blogspot.com/2010/03/gary-gygaxs-whitebox-od-house-rules.html
Malevolent and Benign by Expedious Retreat Press
Petty Gods Revised
Jason Vey's Conan for OD&D 0 Free Game https://www.grey-elf.com/
Fantastic Medieval Campaigns by Marcia B
Varlets and Vermin
Zenopus's One Hit Monsters List https://zenopusarchives.blogspot.com/2013/02/one-hit-point-monsters_23.html
The Alternate Wandering Monsters List https://beyondtheblackgate.blogspot.com/2012/08/alternate-bx-s-wandering-monster-tables.html
The Majestic Wilderlands by Robert S Conley
Supplement X Philotomy's Musings
The Black Sword Hack Ultimate Edition by The Merry Mushmen
Worlds Without Number - Sine Nomine Publishing
Lavender Hack by Ziapelta Games
For Gold & Glory by God Emperor Games
OSRIC by Stuart Marshall
Swords & Wizardry by Finch - see new revised edition
Supplement v Carcosa by Geoffrey Mckinney
The Perrin Convention - get the pdf of All the World's Monsters II it is inside
of course Chainmail 3rd Edition and it's first 5 supplements - Men & Magic, Monsters & Treasure, The Underworld & Wilderness Adventures, Greyhawk, and Blackmoor
Please Post Up Your List
I tend to focus on products which go against certain "maxims" of play; having alternatives to some of the more strict rules inherited from Gygax offers insight into the philosophy behind certain rules and whether such rules are actually fun at the table.
The Black Hack is my go-to book for this purpose: distances are relative, consumables are abstracted to the usage die, experience is based on stories told and not treasure dragged back to town, and all of the dice rolling can be made by players if the GM so chooses. Such a free system allows for easier hacking; You don't need to compare relative power of classes when determining how much XP your homebrew needs in order to level up.
For supplements and splatbooks, I particularly like Wonder & Wickedness and Marvels & Malisons as magic supplements. It's easy as hell to slap together a sorcerer class from these two books and staple it onto the Black Hack. My favourite setting books are Ultraviolet Grasslands and Hot Springs Island; both offer perfect sandbox adventures for the Black Hack. Both offer some manner of departure from the traditional tropes of TSR adventures. I have had to modify some aspects of the latter, such as the
spoiler
miscarriage statuette, which I revised as a fertility amulet.; if worn one way up you're guaranteed fertility, the other way you're guaranteed not to fall pregnant.There are a stupid number of blogs and zines that I could namedrop, but the one I find myself agreeing with the most is The Alexandrian.