National Science Foundation - The NSF has an REU database that is searchable by either area of research or geographic area. Start here!
Maine Space Consortium Webpage lists opportunities for students from grades K-12 up to graduate students.
The Nucleus - This site has many listings for REUs, along with other resources for physics and astronomy undergraduates. There is a Research Listings search box in the upper left corner of the home page.
Michigan State University Library - The MSU library maintains a searchable database of funding opportunities for individuals, including both research support and scholarships.
Department of Energy - DOE Scholars Program: The DOE Scholars Program offers unique opportunities that introduce students or post-graduates to the agency's missions and operations. Appointments are available in a variety of disciplines at participating DOE facilities nationwide.
Pathways to Science - The Institute for Broadening Participation (IBP) has a searchable index of over 650 paid summer research and internship programs in the STEM fields.
NASA - The Office of Education Infrastructure Division Launchpad has searchable student opportunities such as fellowships, internships, and scholarships as well as information regarding recruitment events.
The American Physical Society has a list of undergraduate (and graduate) fellowships.