Summer Research is an excellent way to acquire a strong background for future graduate studies.   

National Science Foundation
NSF’s Research for Undergraduates (Research Experience for Undergraduates, aka REU) "sites" are established in all field of science, mathematics, and engineering. Opportunities for undergraduates to join research projects are offered each summer at major universities.

College Journal
CollegeJournal.com is the premier free site for undergraduate who want job-search and career-guidance information.

U.S. Department of Energy 
U.S. Department of Energy labs recruit summer interns and hire full-time employees in engineering and the sciences.

Science Jobs
ScienceJobs.com is part of the world's leading science and technology jobs service, offering placement in the fields of biology and chemistry.

Kelly Scientific Resources
Kelly Scientific Resources matches top scientists with top companies. The company enjoys a leading reputation in the scientific staffing industry.

Engineering Jobs
Engineeringjobs.com offer job listings for engineers and technical professionals, which you can browse by state.