Lists and Directories of Summer Research Opportunities:

National Science Foundation (NSF) - Research Experiences for Undergraduates (REUs)

NOTE:  Please also see Resources for Students Applying for REUs.

National Institutes of Health (NIH) Summer Internships

American Association of Medical Colleges (AAMC)

Faculty for Undergraduate Neuroscience (FUN) - Neuroscience Internships Listing

American Physiological Society (APS)

SACNAS (Society for the Advancement of Chicanos and Native Americans in Science)

Boston College EagleLink Service (sign in required)

Specific Programs:

Amgen Scholars Program
Note: Amgen has multiple sites available at research universities. Application deadline depends on specific site.

Centers for Disease Control and Prevention (CDC)

Columbia University IGERT on Optical Techniques for Actuation, Sensing & Imaging of Biological Systems

NASA Undergraduate Research Program

Thomas Jefferson University Research Program (Philadelphia, PA)

The Jackson Lab - Summer Biomedical Research (Bar Harbor, ME)

Baylor College of Medicine - SMART program
Summer Medical and Research Training - Houston, TX

Sackler Institute of Graduate Biomedical Sciences, NYU School of Medicine

Cincinnati Children's Hospital - Summer Undergraduate Research

Johns Hopkins University Summer Research

Mayo Clinic Summer Research Program (Rochester, MN)

Cold Spring Harbor (NY)

Nemours / Alfred I. duPont Hospital for Children (Wilmington, DE)

University of Texas Southwestern Graduate School of Biomedical Sciences - SURF program

University of California, San Francisco Summer Research Training Program

University of Wisconsin, Madison Summer Research Program in Biology

Massachusetts Biotechnology Council
Note: search by "internships" -  offered in the summer and during academic year

Environmental Sciences:

Environmental Protection Agency (EPA)

United States Department of Agriculture (USDA)

NSF REU sites focusing on Ocean Sciences

Abroad:

RISE (Research Internships in Science & Engineering) (Germany)

John Innes Centre and Sainsbury Laboratory (United Kingdom)

Institut Pasteur (Paris)