Simply Hired is a mega-site with between 4.5 and 5 million jobs. Type in the internship you are interested in (Marketing Intern) and then type in the city and state you are interested in.
Monster Track is owned by Monster.com but is a separate database specifically for college students and graduates with internships, jobs and other valuable career information.
Idealist is a non-profit agency that promotes opportunities in the not for profit sectors.
The Washington Center is a great site if you are interested in government related internships and others.
The National Science Foundation has opportunities for Research Experience for Undergraduates (REU)