Professional Organizations
Canadian Mastiff Club
Canadian Kennel Club
Mastiff Club of America
Orthopedic Foundation for Animals

Informational Links
Devine Farm - Articles & Info about Mastiffs, as well as the best online stud dog registry I've ever found!
Clicker Solutions - Clicker training!
Show Dog - Super site about everything show related
Dog Owner's Guide - A guide to everything "dog"
Mastiff Online Community - A chat group that focuses on the "whole" Mastiff. Lovely people!

Breeders
Banda Perm Reg'd
Fortress Mastiffs
Gallant Mastiffs 
Spillers Mastiffs of Audley Farms