The following is an ever-growing list of recommendations.
I'll try to keep it up-to-date going forward, but please let me know if I've missed anything!
Systems
- 3d6 In Order
- Adventures in the Hundred Kingdoms
- Barbarians of Lemuria
- Basic Fantasy Project
- Blood & Treasure
- Bloody Basic
- Bluehack
- Dark Places & Demogorgons
- Dark Streets & Darker Secrets
- Everyone is John
- Fantastic Medieval Campaigns
- Kuf
- Labyrinth Lord
- Love in the Time of Seið
- Low Fantasy Gaming
- Microscope
- Monolith
- Mydwandr
- OSRIC
- Oneironaut
- Ordure Fantasy
- Pocket-Sized Fantasy
- Quill
- Rad-Hack
- Rats in the Walls
- Relics & Ruins
- Retropunk
- Romance of the Perilous Land
- Rovers & Riches
- Solar Blades & Cosmic Spells
- Star Dogs Referee's Handbook, Player's Handbook
- Sojourn
- Stay Frosty
- Swords & Wizardy
- The Deep Dark
- The Hateful Place
- The Nightmares Underneath
- The Shadow of Yesterday
- Veil 2020
- Vieja Escuela
- Warrior, Rogue, Mage
- Warriors of the Red Planet
- White Box
- Whitehack
- Yokai Hunter's Society
Adventures & Settings
- Advanced Adventures Compendium 1
- Adventure Anthology 1 & 2
- Adventures on Dungeon Planet
- Anomalous Subsurface Environment (also 2/3)
- Arizona Goes to Hell
- Basilisk Hills Ultimate Hexcrawl
- Crypt of the Lilac High Priest
- Deep Carbon Observatory (first edition)
- Demonspore
- Descent Into The Caves of the Unknown
- Desolation of the Black Terror
- Eilean Dubh: The Black Isle Gazetteer
- Fire on the Velvet Horizon
- Geir Loe Cyn-crul
- HS2 The Forsaken Burial Vaults
- Hubris: A World of Visceral Adventure
- Into The Wild
- Mission Excalibur
- Monkey Isle
- Monkey Murder Mansion
- Morgansfort: The Western Lands Campaign
- Old School Adventures Module TM1: The Ogress of Anubis
- Planarch Codex: Wintry Plane & Dark Heart
- Planet Eris Gazeteer
- Rise of the Blood Olms
- Ruins of the Undercity
- Saga of the Giants
- Secrets of the Wyrdwoode
- Space Wurm vs. Moonicorn
- Spores of the Sad Shroom
- Starter Adventures
- Stonehell: Down Night-Haunted Halls & Into The Heart of Hell
- The Blackapple Brugh
- The Blasphemous Roster Guilds of Infinigrad and their Machinations
- The Chaotic Caves
- The Forest of the Treelords
- The Greydeep Marches
- The House Under the Moondial
- The Hyqueous Vaults
- The Mortuary Temple of Esma
- The Nameless City
- The Night Wolf Inn
- The Ruined Hamlet / Terror in the Gloaming
- The Sepulchre of Seven
- The Vanilla Adventure
- The Undermall
- Times That Fry Men's Souls
- Tomb of the Sea Kings
- Towers of Krshal
- Valley of the Five Fires
- Wyvern Songs
Game Tools
- A Guide to Thieves' Guilds
- Basic Fantasy Field Guide
- Burgs & Bailiffs
- Class Warfare
- d30 DM Companion
- d30 Sandbox Companion
- Deadly Dungeons
- Fifty Fiends
- Filling in the Blanks
- Kefitzat Haderech
- Old School Adventures Accessory CC1: Creature Compendium
- Petty Gods
- Play Unsafe
- Story Games Names Project
- The Dungeon Dozen
- The Kontext Spiel Collection
- The Lazy Dungeon Master
- The Metamorphica Revised
- The Wilderness Alphabet
- Beasties: 1 & 2
- Dyson's Delves 1 & 2