Such a little thing, but: the Rules Reference card in introductory-level Magic: The Gathering products. Magic's full rules are 250 pages of legal-lite text, and

Such a little thing, but: the Rules Reference card in introductory-level Magic: The Gathering products. Magic's full rules are 250 pages of legal-lite text, and yet, this little folded insert, with just five cardface's worth of text, amply covers everything a complete beginner needs to play. It's a brilliant example of cutting an explanation to the bare minimum and piggybacking on background knowledge.