General job sites like Indeed and Wagner Works are great places to start your initial internship search, but it's also important to find other websites and job boards that focus more specifically on your chosen field:
Arts and Entertainment
mandy.com - An international film and TV production resource, including film and television production jobs and casting calls
New York Foundation for the Arts
Business - Accounting, Finance, and Marketing
AICPA Job Board - The American Institute of CPA Job Board
American Marketing Association
Education
National Alliance for Public Charter Schools
New York City Department of Education
New York State Association of Independent Schools
Fashion
Government
Internships in NYC Government - Links to the various internships available in New York City government offices, including the Urban Fellows Program, the NYC Service Fellows Program, the Public Service Corps (PSC), and the Citywide Summer Internship Program.
New York State Department of Labor
U.S. State Department Internships - The U.S. Department of State offers two programs for high school, undergraduate, graduate and post-graduate students who are interested in working in a foreign affairs environment: The U.S. Department of State Internship Program (unpaid) and the Pathways Program (paid).
Health Care
HealthECareers
Global Health Council
International
Journalism, Publishing, Communications & Media
The Public Relations Society of America
Law
The New York County District Attorney’s Office
Richmond County District Attorney’s Office
Non-Profit/Public Service
Idealist.org Comprehensive listings of jobs and internships with non-profit organizations, as well as substantial volunteer opportunities
The Association of Fundraising Professionals
The Chronicle of Philanthropy Job Board
Dot.Org.Jobs - A job and internship board published by onphilanthropy.com
The Nonprofit Jobs Cooperative
Opportunity Knocks - job and internship postings with nonprofits, as well as advice on developing a non-profit career
Psychology
Sports, Recreation, and Fitness
NACDA Internships - The National Association of Collegiate Directors of Athletics maintains a listing of internships at athletic departments at colleges and universities around the country.
STEM Fields