.Hack (or Dot Hack) is a anime and video game series that focuses on the players of a fictional MMORPG known as THE WORLD. A strange virus infects the game and causes players to fall into comas and sometimes their mind ends up being unable to log off the game.

The series started with the anime .Hack//Sign:

The World has become the most successful online game ever, played by millions of people across the globe. But none of that concerns Tsukasa - all he wants to do is log off, and he can't! He seems to be a virtual prisoner inside the game - which, as he discovers, gives him some rather unique abilities. During his wanderings, he meets a variety of characters and soon realizes these encounters may be his only chance to break free of the game

from there the story begins and continues in the PS2 4 game series starting with .Hack//Infection:

Kite, a new player along with his high level friend end up in a strange field and notice a young girl being attacked by a strange monster who has Infinite HP. Watching his friend's avatar get destroyed he discovers that in the real world, his friend has ended up in a coma. Seeking answers he teams up with various players and with the use of the strange ability known as Data Drain which allows him to rewrite data, he becomes the center of a event of epic lengths. In the end, he and his friends end up becoming such legends that their story becomes a part of the game as the legend of the .Hackers (which is refereed to in the later series)

There are more games and series but those two are the ones I'm most familiar with.

.Hack is incredibly deep and it's very easy to see that allot of thinking was put into the back story.

It easy to miss the connections to the other series if you don't pay attention or haven't watch them all. an example is the fact that one of the support characters from the G.U. games was Elk from .Hack//Infection and that he and his cat played a vital role to the back story in the oavs

Not for the faint of heart (the plot will confuse you if you dont watch and play the ENTIRE .Hack series)

but I recommend at least checking out Signs or Infection.

The music is awesome and sets the mood real well and the games act just like an MMO (with the oh so anger inducing level grinding) and each game lets you carry on your save to the next one once beaten! (which makes things easier if your already Lv.60 when you start the first lv in pt2 or 3 cause the monster are STRONG!!!!!!)