Boardfree — Longboarding 6000 miles of road in 2006. A great adventure for a great cause.
Silverfish Longboarding — for everything longboarding (sliding, bombing, slalom and pie)
NCDSA — Great longboard forums. Not as busy or as slick as silverfish, but still a lot of useful info.
Skogging — New schoolers may not like pushing mongo, but why limit yourself to one stance? Chris Yandall is the ultimate resource on Skogging. I've lost a lot of weight through doing this and had a lot of fun in the process :).
Skumping — Skogging is long distance balanced pushing; with Skumping you can travel the same distances without ever touching the ground. Hard work, looks... ok, it looks pretty silly, but the sense of achievment when you manage to cover the distance is amazing. (I'm only managing 1/2 mile skumps at most and that leaves me pretty stoked; can't imagine how the guys managing marathon distances are feeling).
Surface Motion Skate — Techniques for riding big boards. Everything from how to drop knee turn to ollieing a 5 footer. Essential site if you skate more than 40".
Dancing — Watch the videos and gape in awe. That's all I have to say.
Lush Longboards — UK Based Longboard manufacturer. Nice boards, great customer service and an active UK Longboarding forum too.
Faltown Skateboards — Another UK manufacturer, but these guys are focused on sliding. Good trick tips section.
G F Hurley — A US based manufacturer with some real nice looking stained wood finished longboards alongside some killer minis. And they're real helpful too.
Longboard Larry — Beautiful hand shaped boards. Made to order and can be customised for your weight, flex preference, riding style, shape preference... You name it and Larry will build it. I've gotta get me one of these. Or more.