.hack//Liminality and Online Jack are two unusual elements that are included in the .hack and .hack//G.U. videogames. They fall somewhere between in-game animated FMVs and an independant animated series designed to tie-in with a videogame.
.hack//Liminality is a series of four animated episodes, each on a disk that is packaged with one of the four volumes of .hack, and they range from about 15-20 minutes in length to 45 minutes. More importantly, they are mostly unrelated to the characters and main action of the games themselves. rather than tell the story of the games, they tell the story of people other than the game characters who experience firsthand the damage that originates from the game's conflict, and those characters' attempts to understand that conflict. The games take place entirely within the fictional MMORPG "The World", but .hack//Liminailty takes place in the real world. The degree of separation is such that only passing references occur in the games to the events of the animated episodes.
The .hack//Liminality episodes' main purpose is the portrayal of why it is important that the heroes of the game succeed in their goal. Episodes 1 and 2 in particular show people who are put in danger and who have a common friend who has fallen into a coma due to the problems found in "The World". This actually helps the games themselves quite a bit, because the way the plot and set-up of the game itself prevents the player from seeing these problems firsthand while playing the game. Also, episode 3 helps clarify a lot of the otherwise vague backstory of the game. Episode 4 is a lot less effective, because it doesn't serve such a clear purpose for the game's story.
Online Jack from .hack//G.U. is in a very different format, as a set of optional movies that can be accessed within the game itself. What is more, it is broken into a larger number of episodes, each of which is unlocked by progressing within the game story. This connects it more closely to the game's timeline, and integrates it with the various simulated forums and animated news boardcasts that provide all kinds of information to the player about the game world. This reduces the ability of the episodes to tell a long, self-contained story (which .hack//Liminailty Episode 1 did very well), but it does make the episodes more closely conected to the game experience itself.
The problem with Online Jack is that the events of the episodes do not tie in closely enough to the main game's plot. Online Jack portrays several characters who are trying to gather information about certain events, but, for the most part, the player knows far more about what is going on than those characters do, so it contributes very little to the plot of the game, and results in a plot that feels incomplete.
As a whole, I think the concept behind .hack//Liminality and Online Jack has a lot of potential, and could probably be used more widely. It seems that it is very difficult to strike a balance between the need to connect such a side-story in with the main plot and to keep it entertaining in its own right, but if it is accomplished succesfully, it cna probably add a lot to a complex story.