Bookjobs - http://www.bookjobs.com/
A one-stop shop that posts entry-level jobs and year-round internships from over 35 book publishers. A valuable tool for both undergraduates and post-graduates seeking a stimulating career path.
Environmental Career Organization - http://www.eco.org/
Provides information and links related to environmental career services including job and internship information.
Idealist - http://www.idealist.org/
Hundreds of nonprofit job and internship listings.
JobNet - http://www.westga.edu/~coop/
A website of websites which post internships.
Kalamazoo College - http://www.kzoo.edu/career/
Website that provides links to numerous internship oriented websites.