Unearthed Arcana

In two parts, the first of which was a Players Handbook supplement of additional character races, classes, equipment, and spells for Advanced Dungeons and Dragons; and the second section of which added some new magic items, and some esoteric discussion of the stuff in the first section, ala Dungeon Masters Guide.

One of what I consider the (inferior) "second wave" of AD&D rule books.

In Third Edition D&D, or, more specifically, 3.5 Edition, there is a new version of Unearthed Arcana which is essentially a compendium of variant rules. It includes such things as the Vitality/Wounds alternate Hit Point system from the Star Wars RPG, lots of new modified classes such as chaotic good, lawful evil and chaotic evil paladin variants and domain-casting for wizards and druids. While no Game Master could ever be seriously expected to include all, or even most of its variants, a few of them can seriously spice up a game - or break it, if you're not careful.

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I would heartily recommend this book to any serious GM - if nothing else, it is a good source of unusual ideas.