The GHA is sort of a fan club for Halloweenies. They also have their own quarterly magazine. Most importantly, they hold the annual Halloween Convergence.
Grim rides is a car club for Hearses located in the San Francisco Bay Area. Even if you don't live on the left coast, they have links to other clubs and everything hearse.
The HHOA is an organization geared towards the basic DIY home haunter. They also put out their own mag three times a year called Banshee Press. T-shirts and ball caps with their logo are also available.
At Transworld 1997 Leonard Pickel founded the first trade association of the haunt industry -- the IAHA (International Association Of Haunted Attractions), a trade association primarily geared towards the commercial operators and suppliers. Although some don't understand the value of or don't trust the intentions of such an association it promises to become a driving force in the industry.
Speaking of Transworld, they have the big halloween show in Rosemont, Ill every year as well as another show in Sin City. There is no 'official' web site for these conventions, but if you plug a few phrases (such as "transworld 2002", "transworld pics", "transworld halloween", etc.) into your favorite search engine you'll see what it is like at the coolest convention you'll ever attend without having to wear a fez and sell buttons.