Cornell's Legal Information Institute Probably the most expansive legal reference tool online. Useful features include Law About with links to every legal topic imaginable, constitutions and codes, court opinions, law by source or jurisdiction (the 50 states and DC), and the American Legal Ethics Library. Key features are The United States Code, U.S. Supreme Court (1990-date), and New York State Court of Appeals opinions.
Find Law Best legal information starting point for the consumer. Serious researchers can find Supreme Court Decisions from 1983-date.
First Gov This site offers a powerful search engine that searches every word of every U.S. government document in a quarter of a second or less. Includes a separate search engine for the United States Patent and Trademark Office (1976-date).