The American Association of Equine Practitioners
www.thehorse.com
The AAEP is an organization of equine veterinarians that is dedicated to furthering research into equine medical topics as well as furthering horse owner education. This is their horse owner site, and it is very well done (and not just because I write for them!)

Florida Horses
www.floridahorse.com
This site has a lot of information including training, management, a great section on Southeastern toxic plants, and places to ride that are nearby. Many links to competition associations as well.