REUs
American Mathematical Society list of REU sites
Mathematical Association of America list of REU sites
Summer programs for women in mathematics
EDGE Program Summer program at Bryn Mawr and Spelman, for women who are applying to graduate programs in mathematics.
Summer Mathematics Program For Women Undergraduates At Carleton and St. Olaf Colleges.
Summer Program for Women in Mathematics At George Washington University.
Internship opportunities
AMS internship and co-op opportunities
INFORMS summer internship service