Supernatural On the WB
A group devoted to discussing, analyzing, cheering for and complaining about the show.
Winchester Writers
Fanfic group for those interested in reading and sharing stories, dribbles, scenes and plot bunnies, aka as story ideas as well as having those critiqued.
Supernatural Wikipedia
Everything you might ever want to know about Supernatural gathered together by insane fans with too much time on their hands.
Supernatural Screencaps
For the fan looking for every image ever broadcast.
Fanfiction.net
Where to find the good, the bad and the VERY ugly of the fanfiction written. This is probably the single most used website for posting fanfic, and because of this the quality range is huge. It can be hard to filter through the dross to find the gold. The specific communities can be really helpful for narrowing this down.
The Fan Whisperer
A series of articles written about fanfic by a someone who writes it: why we read it, why we write it and why we really are perfectly normal people.
There are MILLIONS of LiveJournal pages, but I personally hate LiveJournal because it is useless for actually being able to find stories, which is what I'm usually looking for.
