The links below are ones I find helpful and frequently recommend to students. Click on the underlined portion to connect to these resources.
Sites for finding REU's, summer positions, and grad school positions
American Fisheries Society (AFS) jobs board - This is site is more focused solely on aquatic research and a good resources for jobs in this field.
NSF REU search engine - This site provides a list of NSF funded REU's (152 in just biology) and allows the user to search and sort.
Undergrad research advice
Advice on applying to graduate school - This is a great article providing advice on how to apply to graduate school . It contains a good description of the role undergrad research plays in that process.
ESA advice for aspiring ecologists - This is an extensive page run by the Ecological Society of America that contains many links related to undergrad research, opportunites for getting summer experience, and jobs in the field of ecology.