Unpaid summer internships and research assistant positions during the school year are available at several psychology labs. Visit this link to see which labs are recruiting for a given semester and visit the lab websites (listed below) to see if summer internship positions are available.
http://minddevlab.yale.edu/
http://www.socialcogdev.com/
http://cogdevlab.sites.yale.edu/
http://davidrand-cooperation.com/lab/
http://caplab.yale.edu/
http://campuspress.yale.edu/infantlab/
For students seeking a paid research assistantship, the Office of Graduate Student Development & Diversity offers fellowships to undergraduate students seeking summer research opportunities through the SURF (Summer Undergraduate Research Fellowship) program.