National Arborist Association http://www.natlarb.com/
The trade association of commercial tree care businesses. Site has information (for consumers and for professionals) and links on tree care practices and standards.
International Society of Arboriculture http://www.isa-arbor.com/
Arboriculture On-line was developed to assist tree care professionals in developing and maintaining effective plant health care programs. Includes an extensive collection of urban forestry links.