Web resources (any edition)

Online SRDs

Swords & Wizardry SRD

D&D 5e SRD

Pathfinder SRD

D&D 3.x SRD

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Miscellaneous

Dungeonographer (Dungeon drawing software)

Hexographer (Hex map drawing software)

Mad Irishman (Character sheets galore)

Mithril & Mages (Randomizers)

PBE Games (Randomizers)

Printable paper (hex etc)

Wizardawn (Randomizers)

Fantasy City Sewer Generator (Lizardman Diaries blog)

Donjon (lots of random generators for old D&D, 3.x and 5e)

The Lair (Lots of random tables and new monsters for LL)

5e PDF listing on Reddit (List of free legal PDFs)

OSR/RPG Blog List (from the Dyvers blog)