Jobwich - Provides listings of internships in NYC and NJ.
Regional Internship Center - Internship listings in SW Pennsylvania.
BackDoorJobs - Lists perennially unique internships, seasonal and summer jobs, overseas work, volunteer vacations and other extraordinary short-term job opportunities (from a few days on up to a few years!).
Doors of Opportunity - Compendium of over 130 summer opportunities for undergraduate students of color in a variety of industries and locations.