FANDOM
Supernatural Wiki - a great resource for scripts, canon, minutiae, lists of conventions, you name it. I'm fairly sure they have a breakdown of what clothes the boys wore in each episode, seriously. Link will take you off lj.
Winchester Bros - another great rescource for canon, articles, fandom news, etc. They also host 'Winchester Radio' and are involved in a lot of fundraising efforts (bigpretzel linked to them for the Hurricane Sandy Relief auction) Link will take you off lj.

The CW Network Supernatural Page - U.S. episode schedules, previews, and streaming of the most recent eps Link will take you off lj.
FOR WRITERS



spn_bigpretzel's own BETA post

Monsterpedia & The Bestiary - If you are looking for a cool monster to write about. Links will take you off lj.
The Weapons Box - Site dedicated to identifying the weapons used on the series. Make your fic feel more "real" ;) Link will take you off lj.
FOR ARTISTS




Striped Wall - Screencaps: Seasons 1-3 Link will take you off lj.
Oxoniensis - Screencaps: Seasons 1-5 Link will take you off lj.
Angel in Chains - Screencaps: Seasons 6-8 caps here. plus convention pics, news, etc. Link will take you off lj
Screencapped.net - Screencaps: Season 1, 6, 7 & 8 Link will take you off lj.
Repro Images - Behind the scenes, official stills, promos all HQ (you have to register and join the site) Link will take you off lj.
Screencaps/SPN locations (all seasons/episodes) Links will take you off lj.
http://supernaturalfansonline.com/galle
http://ladymanson.com/galleries/tv/TVSu
http://www.supernaturallocations.co