American Society of Appraisers
The nation's oldest multidisciplinary professional appraisal society. Free, but registration is required. http://www.appraisers.org/
IAAO
International Association of Assessing Officers - an educational and research body for people with an interest in property taxation.
NCREIF
Nat'l Council of Real Estate Investment Fiduciaries represents investment managers, accountants, appraisers and other real estate professionals.
Foundation of Real Estate Appraisers
FREA offers benefits and education to appraisers. Formed in response to Title XI of the Financial Institutions Reform Enforcement Act of 1989.