Wizards of The Coast is releasing a pdf version of basic in an incremental release over this summer. It will be (according to press releases) as stripped down but entirely playable version of D&D. Some people consider it a simple marketing ploy, others see it as a boon to the hobby. I see it as the owners of the Dungeons and Dragon brand are finally catching up with the rest of the RPG market. There are a number of fine games out there that offer free versions in pdf form and have been doing so for several years in some cases:
Lamentations of the Flame Princess, or just LotFP.
Is a darker take on D&D-like RPGs supported by a small publishers that puts out adventure modules that range widely in taste and style. I’d recommend it for players more familiar with D&D as some elements do need a modestly experienced DM to do well. It’s certainly worth a look and it’s spin on the classic magic system is good.
Basic Fantasy Role-Playing Game
A well written recast of classic RPG rules with a few minor tweaks. It uses an ascending AC system where AC goes up and there are options to add details to the game that don’t bend it out of shape. It’s a very solid fanstay adventure game and I used it mysekf to run a series of modules from olf classical TSR ones all the way up to modern release with little issue. Neophytes and veterans could find it useful.
This recasting of classical RPG follow pretty closelt to the early/.mid eighties versions of D&D. It is a commercial relase by the small publisher Goblinoid Games but the no-art version of the pdf is free and includes the whole game. There are a large number of free or cheap expansions by other publishers and fans out there. Excellent for veterans and neophytes wanting to play a classical fantasy RPG.
Seven Voyages of Zylarthen
A reconstruction and expansion of the original D&D rules that covers ground forgotten or underplayed by many other versions of the game. The books are inexpensive but the whole thing is also free in pdf. Old-timers will note the similarities and spot the expansions and newbies will have a playable version of the game similar to it's roots.
The old school reference and index compliation started out as a means to distribute rules and publish adventures compatible with AD&D without violating copyright. It stands on it’s own as a good solid game that anyone who played AD&D in the past will recognize and be comfortable with.
Epees & Sorcelerie
Started out as a reworking od classic D&D in French, it’s available in English too. This version of the game is very old school and goes it’s own direction somewhat but it presents an interesting and useable version of the classical game.
An excellent retro clone of D&D in a few different edtions. You can get the simple less complicated Whitebox versions, the Core Rules, or Swords & Wizardry Complete for free in pdf. I use Swords and Wizardy myself as a sort of default version of D&D-like RPG here in this blog a good deal of the time.
There are many other excellent games out there that are available for free or have a free basic version that cover D&D-like adventuring. Do some web hunting you are sure to find them.