For myself (who started playing in the middle history of D&D products) reading OD&D and Chainmail feels like the unearthing of a Rossetta Stone. It's doesn't seem whole, and maybe seems a little broken to modern eyes - a chaotic set of rules allowing for interpretation and certain amount of freedom in play. Many strange assumptions in role-playing games are explained by the presence of these early rules, such as Character Alignment being born out of the grouping of fantasy armies (see Chainmail). What's clear from the very start is that the D&D fantasy genre, although heavily influenced by Tolkien was a very open mish-mash of monsters and settings. The open settings of D&D are beautifully parodied in the "The Gygax - Arneson Tapes" a parallel gaming history suggested in this article - which spawns the Greek-myths-only Mazes and Minotaurs game (free PDF rules). "It could have all been so different". :) But enough of this introspective retro-cloning post-modern explorations in genre and game mechanics...!
If you're any OD&D player and you use messageboards, or just curious, you'll be no stranger to Dragons Foot Forums where older editions of D&D are king.
A retro-clone "simulacrum" alternative to OD&D is the Swords and Wizardry WhiteBox rules. Basically these are the S&W rules (a D&D retro-clone) reverse engineered to resemble the rules of OD&D. Nice reading it makes too. Here is a very concise D&D-style system open to "house" rulings. Unlike OD&D, it's very "pick-up and play". A fine, but controversial in this context, touch is the S&W one-save-throw feature - which may appeal more to modern players - than the confusing 5 D&D categories (Death Rays and Dragon Breath, spells etc.)
By the way "WhiteBox" is a winking reference to the early packaging on D&D, in case you were wondering. ;) The Aceum provides a comprehensive guide to the early packaging and flavours of OD&D.
The free S&W WhiteBox PDF is available to download at the S&W home site. With it's greyscale pictures it is extremely printer ink friendly.
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Get back to 1974, when the first dungeon masters were young, before bellies and the beards grew wide and long!