Career Alternatives for Art Historians - assembled by Charles Rosenberg at the University of Notre Dame
Association of Academic Museums and Galleries - student membership is only $10. Jobs and internships are often posted on the members listserv AAMG-L. You can also connect with the group via Facebook, Twitter, or LinkedIn.
See also the list of Museum Studies Programs in the US:
The American Alliance of Museums - student membership is $50. You can also connect with the group via Facebook, Twitter, or LinkedIn. Museum jobs and internships of all sorts are posted on the AAM JobHQ site, and you can also post your resume there for potential employers to find. There are many other museum-career resources here too.
College Art Association - student membership is $55. You can also connect with the group via Facebook or Twitter. Art-related jobs and internships in academic settings are posted in the Online Career Center, where you too can post your resume and take advantage of many other career-related and networking resources.
American Institute of Graphic Arts
American Art Therapy Association
Western Museums Association - $35 membership, jobs board, facebook and twitter. Also LinkedIn.
Art Libraries Society of North America
Membership ($50 for students), Facebook, Twitter, and LinkedIn
ARLIS-L, e-mail list.
Visual Resources Association
The Visual Resources Association is a multidisciplinary community of image management professionals working in primarily educational and cultural heritage environments.
Membership ($40 for students), Facebook, Twitter, and LinkedIn
The Association of Art Museum Curators
http://www.artcurators.org/networking/ (lists job openings, internships, and fellowships)
http://www.artcurators.org/ (membership, facebook, twitter, LinkedIn)
Museum jobs board here: http://www.globalmuseum.org/