Starting point for the whole series anime wise.
.hack//SIGN is the story of Tsukasa, a player in "The World" who discovers that they are trapped within the game.
Any background required: No
Books that relate to this series: AI Buster, Zero and LINK.
Anime that related to this series: Intermezzo, Unison, and GIFT.
Essential watch? - Yes. If you don't like .hack//SIGN, it's probably a sign you won't get far into the .hack universe.
Side story for .hack//SIGN focusing on Mimiru and Bear.
Essential watch? - No. Just if you like the SIGN story enough to watch it.
The OVA that released along with the original IMOQ series. Does not require background information to watch, but helpful.
Books related to this series: Another Birth, Zero, XXXX, and GU.
Anime related to this series: SIGN and Legend of the Twilight Bracelet.
Essential watch? - Depends on if you are playing the games or watching it without the reference. If you are playing the games or reading another birth, yes. If not, it isn't essential, but you won't be too lost if you aren't familiar.
A sequel episode focusing on characters from IMOQ and SIGN interacting. Knowledge of the four games and SIGN is encouraged to enjoy it.
Books Related to this entry: Another Birth, Zero, and XXXX.
Essential watch? - No. Only if you like both the game series and SIGN as they are prominently featured.
A parody episode featuring SIGN and IMOQ characters in a unique art style. Without knowing anything about either series, you'll probably be lost. Only for those who really are into the two series.
Essential watch? - No. But it is a fun watch if you're in on the jokes.
Note: Only part of this adaptation of the manga is canon. After the Tanabata event, it's all anime only. Not a required watch, but does not require background information to enjoy.
The story of two players who acquire avatars in a unique event while playing The World.
Books that relate to the series: AI Buster, Legend of the Twilight Bracelet, Another Birth, The Wolf and the Wavemaster, and XXXX.
Anime that relate to the series: Unison
Essential watch? - No. The anime really didn't do anything that reading the LOTB manga wouldn't provide instead.
The Prequel/start of the GU canon storyline. Also the start of R:2 storyline.
The story of Haseo, a new player to the newest version of the World. Approached by Ovan and Shino, leaders of the Twilight Brigade, he joins their unique guild and looks for what may not exist.
Books related to this series: GU, GU+, Another Birth, Zero and XXXX.
Anime related to this series: SIGN, Liminality, GU Returner, GU Trilogy and GU Trilogy Parody.
Essential watch? - Yes, it's essentially a prologue for the GU games.
An adaptation of the GU games that keeps vaguely to the plot. A general knowledge of GU is best when viewing this.
Essential watch? - No. While it tries to be an adaptation of the games, it kind of fails to do the story properly enough for a person to watch it and not play the games.
A parody of the Trilogy adaptation. Knowledge of GU would be best to enjoy the humor of it.
Essential watch? - No. If you like the GU games and the GU franchise part in general, might be a good watch.
An extra episode made before the fourth GU game. Best watched after having knowledge of Roots, GU games, and the trilogy if necessary.
Essential Watch? - No. If you are a huge fan of the GU series, it's a good way to have an ending other than the fourth game included in the recode game series.
The story of three characters in the R:X version of the world dealing with a strange event in the game causing real world problems. Does not require previous knowledge to view, but contains references to past entries.
Books related to this series: Quantum and Quantum+.
Extra episode of Quantum.
A .hack movie that stands on it's own. References to past references may require previous entry knowledge.
Books related to this film: Sora: Sekai no Mukou ni
An OVA related to .hack// The movie.