Avibase is an extensive database information system about all birds of the world, containing nearly 1 million records about 10,000 species and 22,000 subspecies of birds, including distribution information, taxonomy, synonyms in several languages and more. Avibase is a superb resource for birders everywhere. Besides its unparalleled database, the site offers checklists for countries, states, and regions on all seven continents, sorted by taxonomic authority. Avibase is pretty academic, but if you are looking for practically limitless information on birds, check this site out.
Avibase is hosted by Bird Studies Canada, a nonprofit conservation organization dedicated to advancing the understanding, appreciation and conservation of wild birds and their habitats in Canada and elsewhere.