Arcana Unearthed (properly Monte Cook's Arcana Unearthed, ISBN 1-58846-065-7) is a role-playing game created by Monte Cook and first published in 2003. Described as a "variant player's handbook", the 256 page hardcover core rulebook bears many similarities to the Players Handbook of 3rd Edition Dungeons & Dragons, on which Cook worked a few years prior. In terms of rules, Arcana Unearthed is based on the d20 system of Dungeons & Dragons, although due to including rules for character advancement it cannot be an official d20 system product. (It does, however, use the Open Gaming License.)
Two of the most important themes of the game are ritual and character choices. Both the setting information and the game rules emphasize these areas far more than is typical of Dungeons & Dragons products. Two examples are metamorphosis, in which faen become tiny, winged sprytes, and Chi-Julud, in which the wise giants temporarily lose their wisdom to become stronger and more warlike. Both involve rituals, and both also involve choices on the part of both the player and character that are much more far-reaching than those typically faced by D&D characters (especially Spryte transformation, which is permanent and irrevocable). Cook emphasizes that players should avoid archetypes (presumeably referring to the character archetypes of Dungeons & Dragons) and design their characters more creatively. In fact, shortly after the game was released, Monte Cook's website, www.montecook.com, held a character design competition.
The campaign setting for Arcana Unearthed is the lands of The Diamond Throne, a land ruled by giants (in contrast to the human dominance assumed in most other fantasy settings). The setting includes many elements of a traditional fantasy setting, along with a focus on ritual and ceremony, runes, and the magical properties of crystals.
Title confusion 
Arcana Unearthed is not to be confused with Unearthed Arcana, the title of two books published in 1985 and 2004 that provided additional material for use in the then-current version of Dungeons & Dragons. The name Arcana Unearthed was chosen partially as a homage to the earlier of these two books. The announcement of the 2004 version came as an unpleasant surprise to Cook and some of his fans, who believed that they had an understanding with Wizards of the Coast that this title would not be re-used in the near future. (Cook praised the book itself, however, calling the 2004 Unearthed Arcana "Probably the best book [Wizards of the Coast has] released since 3rd Edition began" in the introduction to Monte Cook Presents: The Year's Best D20, Volume 1.)
Arcana Evolved 
Arcana Evolved (ISBN 1-58846-780-5) is an updated "director's cut" of Arcana Unearthed released in 2005. Arcana Evolved is 432 pages in full color. It adds the contents of two previously separate books, The Diamond Throne and the Player's Companion, to the original variant player's handbook, along with new material including a new class and race, many new feats and spells, and rules for higher level characters than were possible in the original book. On the less mechanical side, it also advances the time line of the Diamond Throne campaign setting by several years - years which feature the return of dragons to the lands of the Diamond Throne, reviving an ancient rivalry with the ruling giants.
Additional books 
Malhavoc Press have expanded the Arcana Unearthed universe with a series of books, including the rulebooks The Diamond Throne (ISBN 1-58846-057-6), Legacy of Dragons (ISBN 1-58846-958-1), Grimoire II, and Mystic Secrets (ISBN 1-58846-985-9), and the short story collection Children of the Rune (ISBN 1-58846-864-X). Some other publishers, including Blue Devil Games, have also supported the Arcana Unearthed line. Since the release of Arcana Evolved, Malhavoc has released a number of supporting books specific to the new edition, such as the Spell Treasury, Transcendence and Ruins of Intrigue.