Unfiction, one of the Internets primary hubs for “alternate reality gaming”, is currently one of the most populous mythos communities.  Unfiction has two sub-forums dedicated to the Slender Man Mythos, one dedicated exclusively to Marble Hornets and another to the rest of the Slender Man Mythos.  Unfiction is one of the most common places to discover new stories in the mythos, as “trailhead” threads are created to promote and discuss new works.Please remember to read the Unfiction Rules before getting involved and remember that Unfiction is an “out-of-game” resource and that posting in-character or discussing your own project is prohibited.
The Slender Nation forums are a fan-run site dedicated solely to the Slender Man Mythos.  The smaller member base and more lax discussion rules allow a tight community amongst members and a number of off-topic forums are available for members to socialize.  A number of prominent creators are members of this board.Please remember to read the rules before joining/posting.And don’t feed Jeebles.
Slenderbloggins began as an informative blog analyzing different elements of the mythos and particular series, run by a single writer.  In the past few months, it made the jump to a full blown fan site, with a staff of writers tackling more and more content, an independent domain, and publishes on a more frequent and predictable schedule than the other informative sources.
Like Slenderbloggins, Encyclopedia Slendaria is a single operator blog covering various aspects of the mythos and particular series.  While the updates are much less frequent, they have their own style and a unique perspective that make it an excellent resource.
  The My Immersion! Tinychat.  A video hangout for slender man mythos fans with occasional appearances by creators.
The HABIT’S Angora IRC chat was originally established during everymanHYBRID’s Seven Trials of HABIT event as an in-game communication between players.  While the chat becomes in-game if one of the everymanHYBRID characters is present, otherwise it is an OOG hangout and conversation zone for Slender Man Mythos fans, mainly from unfiction.  It’s open to the public.