Below you can navigate to a series of sites that regularly post internship opportunities, as well as pages devoted to certain types of internships.
Job Boards
Association of Professional Schools of International Affairs – Internship Listings
Daybook.com Internship Listings
Georgetown Law Center, Internships for Undergraduate Students
Idealist.org Internship Listings (Non-Profits only)
Indeed.com Internship Listings
InterAction NGO Job Board (Careers in Relief and Development)
GainPower (Jobs That Are Left)
Pathways to Science Internship Database
Traverse Jobs (listings for jobs impacting policy and social change)
USAJobs.gov (Federal Government Employment)
U.S. House of Representatives Internships (listed on House Employment Bulletin)
Internships by Type
These pages include links to the websites of major firms, organization, department, etc. in each field that regularly host undergraduate interns.