Copyright Clearance Center (CCC)
This nonprofit organization clears copyright permissions. They operate as a broker between the rights-holder and the user of the copyrighted material. Perhaps their most useful service for teaching is their "Academic Permissions Service," which clears copyrights for course packets and classroom use (in many cases, CCC already has permission). They currently represent many thousands of publishers and house 1.75 million titles. The entire approval transaction can be done on-line.