Roleplaying is a social pastime, so the roleplaying hobby is about sharing. A roleplaying session is a game of shared imagination; an RPG scenario designer assumes a gamemaster will share it with players. Nowadays, increasingly, RPG players are sharing new versions of their favorite classic rules systems - retro-clones.
In years past, this generous impulse to share was tripped up by tedious issues of distribution. But then came websites, print-on-demand and PDF distribution through webfronts like Indie Press Revolution and OneBookShelf, as well as the free document-sharing site Scribd. Pirates use the web to share copyrighted games illegally, but some fans instead share their beloved systems legitimately, by recasting them.
Under U.S. copyright law, you can't copyright a game rule, only a particular expression of it. So if my game says "To break down a door, roll 1d20 under your Strength score," you can safely present the same rule by rewording it - say, "Compare your Strength attribute to the number rolled on a D20; if your attribute is higher, you break through the door." As long as you don't copy the original wording, or breach a trademark by creating confusion in the customer's mind, you're fine.
Hence the rise of "retro-clones," faithful rules emulations that never dare name the game that inspired them, nor copy its wording, but positively slosh with its concentrated essence. Though some are for sale in hardcopy, all have free PDF versions, proffered as gifts - and, just as important, as missionary documents.
The retro-clones got their start the same way we all did, with Dungeons & Dragons. You may already know some of the current clones, each based on a particular flavor of D&D or AD&D, such as Swords & Wizardry and Microlite74 (primordial white-box "Original" D&D); OSRIC (AD&D 1e); Labyrinth Lord (red-box Basic D&D); and Basic Fantasy (the massive Rules Cyclopedia).
If you're interested in old-school D&D gaming, a strong community of supporters awaits you online. The indispensable blogger is "Days of High Adventure" columnist James Maliszewski, and RetroRoleplaying, among many other sites, offers links and reviews. The old-school forums to beat are Dragonsfoot and the Knights & Knaves Alehouse (though if you like AD&D's second edition or later, be ready to don Armor of Flame Resistance +5). Read the Quick Primer for Old School Gaming by Swords & Wizardry designer Matthew Finch, as well as T. Foster's "Five things that needed saying." Then buy back issues of Fight On! magazine, download a retro-clone you like, and gather your players.
Not that interested in D&D? Today many classic non-D&D systems are being cloned. Two of them spring from busy Labyrinth Lord designer and old-school apostle Daniel Proctor:
GORE (Generic Old-school Role-playing Engine)clones Chaosium's venerable Basic Role-Playing, the foundation system for RuneQuest, Call of Cthulhu, Stormbringer and several other classic-era RPGs. As written, GORE focuses on horror adventures, but because Proctor has released it under the Open Gaming License, and his trademark policy is quite forgiving, you can freely adapt it and even publish the result.(For comparison, Chaosium offers free Quick-Start versions of Call of Cthulhu and its current edition of Basic Role-Playing, but these are free-as-in-beer, not open under the OGL.)
With Ryan Denison, Proctor wrote Mutant Future, which uses variant Laybrinth rules but captures the gonzo post-holocaust brio of TSR's Metamorphosis Alpha/Gamma World. The Mutant Future foreword discusses Proctor's old-school philosophy: "So long as you are having fun, it doesn't matter which edition you are playing. [...] Some of us just prefer older editions of games. Sadly, a new edition of a game is sometimes released not because it needs to be 'fixed,' but because a new edition is the only way to bring in new revenue."(Proctor had the unmatchable old-school honor of playing in the last D&D session ever run by Gary Gygax. "I won't forget how narrowly my character escaped death from a giant spider," he writes in the Mutant Future foreword, "and the mischievous look Gary had in his eyes when he declared someone needed to roll a saving throw versus poison." How cool is that?)