There are .hack manga
.hack//AI buster talk about how Aura was created and why Balmung and Orca are such coveted Players.
.hack//Dusk
Also known as UDEDEN, the twilight bracelet or legend of twilight. It follows Shugo and Rena's adventures and some other chapters not seen in the Anime.
.hack//Zero
A side story occuring after sign and before infection.
There are 4 games (Infection, Mutation, Outbreak, Quarantine), then the special episode of .hack//sign called Unison ended Kite's and tsukasa's adventures. There is also this bizarre animation called .hack//gift that comes when you buy all four volumes of the .hack games. There's also .hack//fragment which is an online game on the PS2, and talks about fragment the beta game which lead to the creation of the World. Aura was first embedded in Fragment, but I'm giving away too much about the story already. There's also the OVA, Liminality, one episode of which comes in each .hack game. It's moatly about gamers in the real world, and most of it takes place there.