Professional organizations, in all their variety, are important resources for research, teaching, study aids, scholarship and fellowship opportunities, blog sites — you name it.
The two most important organizations for classicists in the United States are:
Society for Classics Studies (SCS)
Archaeological Institute of America (AIA)
Two organizations with valuable resources for undergraduates:
National Junior Classical League
Regional Classics Associations in the US:
Classical Association of New England (CANE)
CAAS (Classical Association of the Atlantic States)
CAMWS (Classical Association of the Midwest and South)
California Classical Association Northern Section
Williams is an institutional member of:
The American Academy in Rome
The American School of Classical Studies at Athens
Also of interest:
Lambda Classical Caucus, a coalition of queer (broadly construed) classicists and their friends and supporters, with a political and scholarly mission. Lambda is affiliated with the American Philological Association, but its membership is open to all. It publishes Iris.
Women’s Classical Caucus fosters feminist and gender-informed perspectives in the study and teaching of all aspects of ancient Mediterranean cultures and classical antiquity. It publishes Cloelia.
Complete list of Classics organizations on-line, from Electronic Resources for Classicists: The Second Generation.