Here is a brief list of searchable sites, however many colleges and universities offer summer enrichment programs for high school students. Your best bet is to google pre-college programs and the name of the university you would like to be at! Many of these programs have early deadlines and are very competitive so plan ahead! Be sure to look into the possibility of financial assistance as well, some programs do offer financial support.
USummer - a free pre-college summer program directory
Teen Life - searchable summer program database
Teen Life Guides - summer programs, gap year programs, STEM programs, online programs, performing and visual arts programs, and more!
FastWeb's Summer Programs for High School Students - good article with links to programs
College Xpress list of summer programs by university - Each school on the list is a link to more information about their program
American Collegiate Adventures - offers programs both in the USA and abroad in a variety of subject areas
Summer Classes at Columbia College in Chicago
School of the Art Institute of Chicago summer classes for high school students
Pre-Collegiate Summer Institutes at Stanford
Boston University Summer Programs for High School Students
Summer Science Institute at Carleton College
Carleton College Summer Programs for High School Students
Loyola University Chicago Summer Scholars Program
The Game, Cinema, & Animation Summer Academy at DePaul University Chicago
Summer Programs for High School Students at Northwestern University