Ok this is my first anime review, so it could be a bit confusing but i'll try to do my best.

All the story is about a kid, called Tsukasa, who's playing a MMORPG (Massively Multiplayer Online RPG). He is cold, shy person (It's kind of like Shinji but less stupid).
Tsukasa wakes up on a game called "The World" without any memories, he cannot remember anything, on the first episode he meets one of his friends called Mimiru, who will be one of his friends on this anime. He found a special item, it told him that he'll be his guardian, that he'll protect him. By this time Tsukasa realized that he couldnt log-out..
Then rumors started to spread out about him, because it seemed like his existence wasnt on the logs. The World was threatned by a Hacker, so, the Guards and their leader Subaru tried to catch him thinking that he might have a connection to him. Then when they sent a guard to get him, suddenly when Tsukasa was in danger, a strange shaped thing appears, and stabbed the guard with soem strange ray that looked like it destroyed the information (U can see his binary code). Then you see a report to Subaru-Sama that it said that the guard they sent to get Tsukasa has fallen into a comma.
The story is way more interesting than this, with lots of cheerful characters, and a superb character design (By Yoshiyuki Sadamoto, character designer from the all known Evangelion anime, and currently writing its manga). It takes a dark turn on the last episodes, and it has a strange ending. (Im not telling you what is it, u gotta watch it :oD )

Its one of that animes you must have. :D It's composed of 26 episodes of about 24 minutes each. Available for download on Kazaa.

There are currently 2 more series about .Hack. Its .Hack//Liminality (4 Ovas, 3 released) and the new .Hack//Legend Of The Twilight Bracelet (6 Released, 5 Fansubed so far, by AnimeJunkies.