"Creative Commons is a nonprofit corporation dedicated to making it easier for people to share and build upon the work of others, consistent with the rules of copyright."
The official website of the United States Copyright Office, a department of the Library of Congress that "registers copyright claims, records information about copyright ownership, provides information to the public, and assists Congress and other parts of the government on a wide range of copyright issues, both simple and complex" (Home > About > Overview, United States Copyright Office)
The Fair Use Index includes a searchable database of court cases related to fair use, intended to be used by lawyers and the public to guide in determining whether their use of something falls under fair use.
The official avenue for registering copyright claims with the United States Copyright Office. Note that this is not required for your work to be copyright protected.
A guide from the United States Patent and Trademark Office to help people determine whether they need a patent, a trademark, or to file a copyright claim.