British Trust for Ornithology - a research organisation which coordinates a number of major surveys, including Atlases, Breeding Bird Survey, Wetlands Bird Survey etc.
Wader Quest - ambassadors for waders/shorebirds. The Gay Birders' Club is a founder club sponsor.
Cornell Laboratory of Ornithology - sound recordings galore, mainly for North American birds, but the labs collect wildlife recordings from across the world
Worldwide birding links
Where do you want to go birding today ? - Probably the most comprehensive set of links for birding sites. I doubt there is anywhere in the world that has been missed out
Seabird sites
Ocean Wanderers - Angus Wilson's identification resource. Includes taxonomic list for all the seabirds and marine mammals.
Brian Patteson Pelagics - a fantastic site giving a lot of information about pelagic trips organised off North Carolina. Lots of pictures, including a new species for the North American list.
Shearwater Journeys Home Page – Debi Love Shearwater's site for pelagic trips out of Monterey Bay, California. Anyone with a name like this has to be into seabirds, big time.
Elsewhere in the world
Australian Birding - a great starting point for information about birding in Australia and a good set of antipodean links