American Association of Museums
American Institute for Conservation
Canadian Conservation Institute
Conservation OnLine (CoOL)
Getty Conservation Institute
Northeast Documentation Conservation Center
Northern States Conservation Center
Packing, Art handling & Crating Information Network (PACIN, a professional interested committee of the American Association of Museums)
Registrars Committee of the American Association of Museums

The Smithsonian Institution maintains a detailed (and very broadly defined) list of art conservation organizations. It can be found here: http://www.si.edu/mci/english/learn_more/taking_care/artorgs.html