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
Business - Accounting, Finance, and Marketing
AICPA Job Board - The American Institute of CPA Job Board
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.
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).
Journalism, Publishing, Communications & Media
Idealist.org Comprehensive listings of jobs and internships with non-profit organizations, as well as substantial volunteer opportunities
Dot.Org.Jobs - A job and internship board published by onphilanthropy.com
Opportunity Knocks - job and internship postings with nonprofits, as well as advice on developing a non-profit career
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.