Without a doubt, I consider the "Single Volume" edition of 0e to be the single most indispensable book at the table for running any 0e, or even a Holmes Basic game.
No, its not 'legal' in the strictest sense of OGL and copyright compatibility, but it is free, and in its favor is that it keeps a lot of the 0e text intact, only changing it when it vastly improves the rules. In fact, I'd say that it is this simple bit of clarity and unchanged text is the reason why its not legal: its so damn useful, and it really makes the creation of 0e retro-clones unnecessary.
Yes, retro-clones are nice when WotC wouldn't release the PDFs of any of the original D&D game versions, and new players needed an actual set of compatible rules to play with; not to mention that new modules need a published set of rules to be published in the first place. So, I think everyone who still focuses on the original versions of D&D really needs a PDF copy of the clone that mimics their chosen game, just in case someone gets the urge to write up a campaign or module, the cloned rules are there to base your game product on.
A lot of times however, a clone gets produced to wide acclaim and sales, but picks up a segment of the gaming population as detractors of the game, claiming that a favorite rule of theirs wasn't included, changed, or other rules they don't care to see was included, making the game nearly unrecognizable, and plays quite different than the original version played.
Now, i don't know a whole lot about any clone, but I know which ones I like and would buy in print, and others I wouldn't take a free PDF of if it was offered. There comes a point when a clone must be published with a discernible difference to be published legally, and that difference is the difference between a great original version of the game, and a collection of house rules tacked onto a rather wordy interpretation of what used to be considered a good game. And why is this necessary? So a publisher can make a few bucks, or to truly enhance everyone's understanding and clarity of the original rules?
I'll say this up front: I've been working on my own retro-clone for years now, but long ago I decided that it wasn't a retro-clone or a revision; it is a set of house rules that I will eventually be playing with (someday, hopefully) and base all of my future campaign material and modules on.
It does three things that other retro-clones cannot, or will not do: It will not be OGL compliant, nor will it avoid any copyright issues; it will not pretend to be 'faithful' to the original it attempts to mimic, when enough is changed to make it no longer so; it will never be released to the public at large, and it will never cost anyone one red dime to get a PDF copy of it.
It will, however: Use original version text, sections, numbers, formulae and terms that the original version used so there is no confusion as to what, exactly the rules are about; it will obviously offer "house rules" based on any number of sources to make it a more rounded, well traveled game; once it is complete, I will most likely print a copy for myself, and if it isn't cost prohibitive, I'll print up extra copies for players and friends. Whether they pitch in to help cover the cost of printing is up to them. Eventually, I have to receive a decent enough paycheck to afford printing up a copy every once in a while.
Here at Prime Requisite, rules will be mended, broken, modified, fractured, opened, sealed, forgotten and invented from whole cloth. New classes, races, equipment, descriptions, monsters, magic items, charts, graphs, tables, spells, worlds, dungeons, and reasons to adventure will eventually be covered, discovered, recovered, visited, revisited, and discarded, and in no particular order.
I'll post more about my "house rules" in my next post.