Participating in a summer research program has many benefits, summarized in From Summer Research Participant to Graduate School Contender.
Listed below are many undergraduate summer research fellowships. To search on your own, Google the word "research" and add in one of the following acronyms: REU (Research Experiences for Undergraduates), SROP (Summer Research Opportunities Program), SURF (Summer Undergraduate Research Fellowships), SURP (Summer Undergraduate Research Program), or MURF (Minority Undergraduate Research Fellowships).
Rochester Institute of Technology provides a list of university-based research positions. These are especially useful for students who want to explore research careers and/or graduate school.