United Cardists - The biggest non-brand community around cardistry, it's section on playing cards is the home of many collectors.
Cardology Playing Card Forum - A new and growing playing card forum with many sections, catering to all of ones playing card needs.
52Problems - A site dedicated to vintage playing card collectors made by Taylor Bomarito aka CardConjurer. Features useful articles and info on vintage decks.
BMPokerWorld (blog) - The blog of a very reputable playing card retailer, updated with info on upcoming decks and other interesting tidbits from the playing card world.
Bicycle Backs Handbook - Mrs. Robinson's Bicycle Playing Card Collector's Handbook
Big Blind Media - The home of the Karnival custom deck series.
Playing Cards Only - A british site selling playing cards and related merchandise.
Counterfeit Bee's - An excellent article on counterfeit Bee cards.
Many of the pictures (green background) are courtesy of Sinjin7, and you can see all of his pictures here
I was lucky enough to get permission to use some great photos of certain decks' faces from the collection of JenX5.
They are marked with her name and you can see all of her photos here