University of Minnesota
University of Minnisota: Radcliffe's Ipm World Textbook
This is a site designed and used by professors to teach integrated pest management. A part of the National IPM network. Gives great photos and countless suggestions.
Cornell University
Cornell University: A Guide to Natural Enemies in North America
In-depth info on four natural enemies: parasitoids, predators, pathogens, and weed feeders. Great identification of target insects and plants.