Swords & Wizardry is OD&D made easy and accessible. It has everything you need to play in one book, from character creation to high level playing, from referre tips and guides to monsters and treasures. Also, it's easy to read, one afternoon is enough to read a book and get your first adventure ready. Character creation takes minutes, just like combat. There are also optional rules, which make it easier for younger audiences to get into the game.
There are of course some errors, but only minor, and the Referee is free to do with them what he likes. Still, I would be grateful, if there was a non-free version of the book in pdf format, which includes the Thief and Monk classes from the print edition - although it's not hard to create your own version of these classes, and there are some gamers, who already did it...