Pedigree Discussions and Forums

About.com message Boards - Contains an area specifically for discussing thoroughbred pedigrees.
Breeder's Digest - Discusses thoroughbred breeding and training techniques as well as a general racing discussion.
Del Mar Pedigree Message Board - Generally used for specific pedigree questions and looking up information.
The Homestretch - Offers some very intelligent breeding discussions. You'll find Steve Roman posting here answering questions about dosage.
Let It Ride Breeding Forum - A lively of group of people interested in a variety of breeding topics.
Thoroughbred Heritage Mailing List - A wide ranging discussion group on breeding, pedigrees, the history of thoroughbreds, and more.