IOC World Bird List - From the International Ornithologists' Union. Includes species, subspecies, and recently extinct species.
World Bird Info - Per Kristof Zyskowski, "excellent for old synonyms."
Avibase - the World Bird Database - From Denis Lepage. Over 12 million records on 10,000 species and 22,000 subscpecies of birds.
Check-list of North American Birds: the species of birds of North America from the Arctic through Panama, including the West Indies and Hawaiian Islands - 7th ed., 1998 From the American Ornithologists' Union Committee on Classification and Nomenclature (North & Middle America). location