Copyright:

To promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries
                                           - US Constitution, Article 1, Section 8

United States Copyright Office, Library of Congress
http://www.loc.gov/copyright/

key publications, including informational circulars; application forms for copyright registration; links to the copyright law and to the homepages of other copyright-related organizations

Copyright and Fair Use Site, Stanford University Libraries
http://fairuse.stanford.edu/

About Copyright, Legal Information Institute, Cornell Law School
http://wwwsecure.law.cornell.edu/topics/copyright.html

Fair Use: 

Fair Use Education
http://www.ninch.org/ISSUES/COPYRIGHT/
FAIR_USE_EDUCATION/FAIR_USE_EDUCATION.html

Four Factors in Determining Fair Use, Cetus.org
http://www.cetus.org/fair5.html

"While fair use is intended to apply to teaching, research, and other such activities, a crucial point is that an educational purpose alone does not make a use fair. The purpose of the use is, in fact, only one of four factors that users must analyze in order to conclude whether or not an activity is lawful."

Center for the Public Domain
http://www.centerpd.org/

"...the Center seeks to call attention to the importance of the public domain and spur effective, practical solutions and responses. Its work is animated by the conviction that new legal regimes, social institutions and transparent technologies must be created to fortify the information commons."