Astronomy
Aviation - Anything that flies
Beekeeping - BeeSource.com
Cars - Car Clubs, Car Reviews
Caves - National Speleological Society
Collectibles
Cooking - Anything that's YUMMY!
Crafts
American Society of Dowsers
Talking Electronics
Games - Board Games, Billiards...
Gardening
Genealogy
Guns
Houdiniana.com
Juggling:
Internet Juggling Database
Juggling.org

Kites
Knife Throwing: ThrowZini.com
Models (Scale models, etc.)

Pen Spinning:
PenSpinning.com
PensPinning.com
Universal Pen Spinning Board

Pets

Radio:
Amateur Radio Repeater
Art's Antique Radios
Ham Radio - Repeater.org
National Association for Amateur Radio
North American Radio Clubs

Railroad Enthusiasts

Rock Hounds:
Mineralogical Society of America

Rocketry:
Apogee Rockets
National Assn. of Rocketry

Scanners:
Bearcat Radio Club & Hobby Radio Stop

Scrapbooking: CreatingKeepsakes.com
Storm Chasers: FaceTheWind.com
Sword Forum International
Trains - Railroad Enthusiasts
Treasure Hunting
BullWhip Enthusiasts
Writers