Various governmental agencies related to intellectual property:

The United States Patent and Trademark Office USPTO
The United States Library of Congress Copyright Office
The European Patent Office
The World Intellectual Property Association
The Australian Intellectual Property Office
The Japanese Patent Office
The Canadian Intellectual Property Office (CIPO)
The United Kingdom’s IP site
The United States Court of Appeals for the Federal Circuit
The Supreme Court collection via Cornell Universty Law School

General websites related to intellectual property:

The American Intellectual Property Law Association
The Franklin Pierce Law Center’s Intellectual Property site
A Yahoo directory

Specific search engines for patents:

The Google Patents Search interface