Great Modules:

Slave Pits of the Undercity (AD&D)

Secret of the Slaver’s Stockade (AD&D)

Assault on the Aerie of the Slave Lords (AD&D)

In the Dungeons of the Slave Lords (AD&D)

Queen of the Spiders (3.5e)

The Village of Hommlet (AD&D)

The Temple of Elemental Evil (3.5e)

Tomb of Horrors (3.5e)

Expedition to Undermountain (3.5e)

Ruins of Undermountain (3.5e)

Ruins of Undermountain II (3.5e)

The Lost City of Barakus (3.5e)

Tomb of Abysthor (3.5e)

Idylls of the Rat King (3.5e)

Revenge of the Rat King (3.5e)

Crypt of the Devil Litch (3.5e)

Quests of Doom (PF)

 

Tables and Generators:

Mother of all Encounter Tables

Mother of all Treasure Tables

Donjon

 

GM Inspiration:

Dungeon Crawl Classics: The Adventure Begins (3.5e)

Dungeon Crawl Classics: The Adventure Continues (3.5e)

Hex Crawl Chronicles 1-6 (PF)

Bard’s Gate (3.5e)

Book of Taverns (PF)

Weapons of Legacy (3.5e)

Bill Webb’s Book of Dirty Tricks

 

NPCs:

Enemies and Allies (3.5e)

Unusual Suspects (PF)

Ultimate NPCs: Skulduggery (currently a Kickstarter)

 

Rules:

Paizo (PF)

PFSRD (PF)

D20SRD (3.5e)

Fillable Character Sheet pdf (PF)

 

Awesome companies:

Frog God Games (Necromancer Games)

Goodman Games

Nord Games