The Nature Conservancy home page. Overall a pretty useful page, not to mention cool looking. These include chemistry, geology, biology, atmospheric studies, physics and much more. These are all summer programs that allow students to work on research projects with graduate students, university faculty, and post-docs. This is the Yale University guide to summer research opportunities in the sciences.
Programs listed include biology, physical sciences, chemistry, earth sciences and others.
Each of these categories has a list of internships and jobs associated with it. Not all of these opportunities are environmentally related, but they all look pretty good. This is an Iowa State University Program for Women in Science and Engineering. There are paid summer internships through this program in the sciences although again I am not sure if there are internships in the environmental sciences.