JLA Watchtower, also called The Watchtower, is a large DC-based LiveJournal role playing group that started in May 2004. It is considered AU, as many of the players are DC fans disappointed with current plotlines and character developments. While occasionally plots at the Watchtower will be based on plots from the comics, canon is considered a guide, not a straitjacket. (Sue Dibny isn't dead anymore, for example.)
Notable to the Watchtower-verse are player-run plotlines that range from the comedic and lighthearted to the tragic and terrible. Watchtower is also notorious for launching its own version of plotlines found in the comics and animated series while placing a unique spin on the events. Perhaps the greatest example of this is the resurrection of Donna Troy. In the comic books, it was an epic battle between the Titans of Myth. In Watchtower, Arsenal led the Titans in challenging Hades for Donna's life, winning despite treachery, cheating, and the (temporary) death of Nightwing. These player-run plotlines further alter the characters and launch future plots. There has even been some cross over with other universes.
Watchtower also boasts an interesting array of original characters, both as heroes and villians. Setting the stage for them is "Wallace" Hugo Anders, an alternate-universe version of Hugo Weaving brought into Watchtower via the Lunatic Cafe. Others include the Zukov siblings, mages distantly related to a succubus; Jack "Black Knight" Slater, a government-made assassin trying for redemption, and Travis Gray, a young pickpocket who is currently apprenticing as a Green Lantern under Guy Gardner's tutlage.
There has even been some cross over with other universes. During the Operation: Chaos plot, there were multiple crossovers with many different locations. Heroes from many diferent places came together to save the multiverse during this plot. While there are no large crossovers like that planned again in the near future, there is always the oportunity for it. In fact, there are little glimpses here and there, allowing for changes to play beloved characters who are not strictly part of the community.
The Watchtower is well-established and includes regulars to the Nexus like Black Canary, Shayera Thal, The Question, etc. The founding members first met in the Lunatic Cafe, and after several months of RP there decided that they wanted a community of their own for more structured scenarios. The cast list includes canon characters as well as original characters. Recently they added a public OOC community (Watchtower fan community) for player-reader interaction.
For those interested in applying for a character, here is the (application link) as well as (a list of currently played characters).