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:

American Classical League

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)
CAPN (Classical Association of the Pacific Northwest)
CCA-S (California Classical Association-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.

For more:

Classics Associations and Research Institutes, a list maintained by the SCS.

Complete list of Classics organizations on-line, from Electronic Resources for Classicists: The Second Generation.