These links include groups and associations who promote horticulture, but don't specialize an a particular group of plants.
International Association For Plant Taxonomy
I'll bet you have often wondered who those folks are that keep changing plant names and why. Now you can visit their website and read the Botanical Nomenclature Code for yourself and try to make some sense out of the whole thing.
International Plant Propagators' Society
This professional society is dedicated to sharing information about all aspects of plant propagation. The meetings and publications of the proceedings is superb. Links are available on propagation, as well as membership information.
Royal Horticulture Society
The RHS is a valuable resource for their publications, including the famed Plantfinder, which is now on-line and can locate any saleable plant in the UK. In addition, you can get the test results from the RHS trial garden...a fascinating site.
The American Horticultural Society
Despite publishing the ridiculous and useless Heat Zone Map, this is an interesting site with the online magazine, "The American Gardener".
The Garden Conservancy
Loosely modeled after the English Garden Trust, this brainchild of plantsman Frank Cabot was designed to assist in the preservation efforts of great American gardens. This is a superb organization, whose efforts are already being felt, thanks to the support of members. The site details each of the conservancy sites and how to visit.