Cleanrun offers a great agility magazine along with many supplies, books, etc. They also have some training exercises and offer a calendar listing upcoming agility trials in the area
www.akc.org
American Kennel Club offers agility for purebred dogs registrable with their organization
www.usdaa.com
United States Dog Agility Association offers agility for all breeds including mixes.
www.nadac.com
North American Dog Agility Council offers agility for all breeds including mixes.
www.asca.org
Australian Shepherd Club of America offers agility for all breeds including mixes.
www.clickertraining.com
Karen Pryor's site with lots of information on clicker training
www.flyingdogpress.com
Suzanne Clothier's site which contains some great articles.
Everyone should read the article, "He just wants to say Hi"
www.powerpawsagility.com
Nancy Gyes site which also contains several great training articles