Jockey Club: Thoroughbred registry for United States, Canada and Puerto Rico
TOBA: Thoroughbred Owners and Breeders Association
The Greatest Game: All about racing Thoroughbred ownership
Pedigree Online Thoroughbred Database: Free pedigrees and a racing discussion forum
Bloodhorse Online: Thoroughbred racing news and an interactive Stallion Directory
Thoroughbred Times: Another online racing news site, also with an interactive Stallion Directory

Rescue Links:
CANTER: Non-profit organization dedicated to providing retiring racehorses with opportunities for new careers after the finish line. Chapters around the United States, providing listings of horses retiring from local tracks.
The Exceller Fund: Created in memory of Exceller, providing funding for the purchase, care, retirement and adoption of Thoroughbreds
Re-Run: Helping ex-racehorses find second careers
New York Horse Rescue: Offer ex-racers and other horses for adoption
New Vocations: Ohio-based adoption facility, offering retired Thoroughbreds and Standardbreds for adoption

Horse equipment, gear, tack & clothing:
Dover Saddlery
State Line Tack

Bits & Bytes Farm: Offers retrained off-the-track Thoroughbreds for sale. Great resource for ex-racer retraining info.
Ex-racers.com: All kinds of info on ex-racers. Includes a discussion forum.