"Dark Dungeons is a free retro-clone role playing game—a game in the style of old-school 1990′s role playing games, back in the days when rogues were thieves, races were classes, and you could start your adventuring career exploring a few basic dungeons and end up travelling the planes as an immortal being of pure divine power.
In international copyright law, it is not possible to copyright actual game mechanics, only the literary and artistic expression of the author(s) of the game when describing those rules. Therefore- providing one is careful to avoid infringing on the copyrighted expression of the original author(s) while doing so- it is possible to replicate the mechanical rules of an old game that is no longer available in order to provide compatibility with the old game’s expansions and adventures; and to replicate the playing experience of the game."
Free PDF release, with hard copies available at cost from Lulu.
A few months after the initial release, Blacky re-compiled the book, cleaning up a number of typos and errors, and changing the to-hit rules. He took out some 'plateaus' in the progression, and went from the THACO concept of early D&D to a 'Base Attack Bonus' akin to D&D 3rd Edition (though Armor Class still goes down).
Each version can be told apart by the color of the title text on the front cover. The first version has a deep red color, while the second version has light orange titles. Further versions are planned to continue using different colors.