Immersive Ink has released the second version of their Delving Deeper reference rules — a 0e White Box clone that many consider to be the closest set of published retroclone rules to the “three little books” that came in the 0e White Box. This new version incorporates the four pages of version 1 errata released last October into the actual rules.
Delving Deeper is a very nice set of 0e retroclone rules. It is very close clone of the White Box set of rules, even to being published in three booklets. One major addition is the inclusion of an optional thief class. Of course, like every other 0e retroclone out there, the authors cannot help but put their personal interpretation of 0e’s less clear rules and obvious omissions into their game, but they were obviously trying to stay very close to the original edition rules.
Delving Deeper is free to download (and even has art, unlike the free versions of some retroclones) and it is almost entirely open game content under the OGL. If you are interested in 0e, Delving Deeper is well worth the download and provides a third choice in the 0e White Box retroclone world, the other major free options being Swords & Wizardry: Whitebox and my own Microlite74 Basic.