Some that I know of:
Rutgers Summer Acting Conservatory: http://www.masongross.rutgers.edu/extension/rutgers-summer-acting-conservatory
Circle in the Square: http://www.circlesquare.org/summer_workshops.htm
South Coast Repertory: http://www.scr.org/classes/acting-intensive-program/acting-intensive-program-curriculum
UNCSA: https://www.uncsa.edu/summer/drama-summer-programs/index.aspx
Northwestern: http://nhsi.northwestern.edu/theatrearts/
The Performing Arts Project (TPAP): http://www.performingartsproject.com/
For New York State residents: http://www.oce.nysed.gov/nysssa/SOT/