Playing in the 70's

Getting in the mindset for very old fantasy campaigns.

Rules & Rulings

PDFs of the booklets. Much better options below.
Fantastic Medieval Campaigns, a faithful recreation of the original text.
Delving Deeper, a re-edited (and slightly clarified) retroclone.
Swords and Wizardry Complete, a pretty fleshed-out retroclone with numerous modifications.
White Box: Fantastic Medieval Adventure Game, a streamlined retroclone with numerous pain points and tables condensed into even a single number, sometimes.
Necropraxis House Rules, what it says on the tin.
Delta's D&D House Rules, what is says on the tin.
FISTFUL OF CRYSTALS, a detailed set of house rules for running OD&D in the author's setting.
Ode to 74, full ruleset based on original game.
The Littlest Brown Book, a compactly formatted retroclone.
The game, as typed and formatted by Bruce Mohler, rehosted here.

CHAOS REIGNS, an alternative combat procedure for OD&D.
KILLCHAIN, a mass combat procedure for OD&D.
Necropraxis Starting Equipment Packs, instead of buying equipment at character creation, roll up a starting pack.

The Gygaxian Style
The Arnesonian Style
What We Mean When We Say “Encounter”, understanding how to interpret the rolls that determine an encounter.
On the Oracular Power of Dice, how old school gaming relies on creating a story from random rolls.
The Classic Dungeon Crawl, a series of posts discussing why the dungeon crawl is a liked playstyle, and its design space.
Proceduralism, on what a procedure is and why it is good for a classic game.
XP for GP: What are the Alternatives?, discussing how to justify XP for GP and alternatives, should you decide it's not appropriate.
Classic D&D: I used to think... Why Old D&D may not be as outdated as you think.

Muster, which discusses the player expectations and referee guidance on running OD&D (or similar rulesets) as a fantasy wargame (as was intended!).
Stocking Nightwick Abbey, a stocking procedure used in a real game.
The Primordial Stack, a historical look at dungeon delving and its roots.
So You Want to Build a Dungeon?, a nearly comprehensive guide of principles and concrete advice for dungeon design.