Some useful links are:

South American Birds:

The Neotropical Bird Club: A site with lots of information on South American Birds. If you're interested, you should join.
Brazilian Ornithological Records Committee: Mostly in Portuguese but if you want the official Brazil List, this is where to find it.
The Cristalino Jungle Lodge: A great place to stay if you want to see a large number of Amazon birds.
Bellavista Cloud Forest Reserve: Details of the reserve and some great photos by Martin Reid.
Boute Expeditions: Paulo Boute organises birding in Brazil, including the Pantanal.
Wild Macaws: Harold Armitage's site on Macaws
Simon Woolley and Julia Casson: A very good Brazil trip report with some excellent photos.
Neomorphus: An excellent site from Joe Tobias with photos, sound and articles on South American Birds
Birds of the Caatinga: Some excellent photos by Hermann Redies from his farm in the caatinga of Ceará.
Save Brasil: This is Birdlife International's Brazilian partner and the site, in English and Portuguese, has some useful information on the threatened birds of Brazil.
Rick Simpson: Rick is a bird guide presently living in Ubatuba one of the best areas in the Atlantic Rain Forest. Check out his website.
Avibirds: This site covers the birds of Surinam with many fine photos, videos and illustrations as well as comprehensive notes.
Ciro Albano: Ciro Albano is a Brazilian ornithologist who also happens to be a great photographer and bird guide specialising in the north-east of Brazil. On this website he presents some of his best photos.

General:

The Fat Birder: This site seems to have everything on birds and keeps very up-to-date.
Bird Families of the World: gives you a world list by family with some photos and recordings.
Birding hotspots around the world: with some great photos.
WorldTwitch: Better for some regions than others but lots of interesting contributions from around the world.
Peterson Online: Mainly North America but comprehensive info and a rich site.
The Hummingbird Society: Seems to be developing to cover South American species as well.
Subbuteo Books: Highly recommended for books on birds and other nature subjects.
Birding Israel: This site is about birds, birdwatching opportunities and birding hotspots in Israel.
Handbook of the Birds of the World: Lynx Editions are publishing the first work ever to illustrate and deal in detail with all the species of birds in the world
Damisela's Electronic Zoo: An interesting collection of photos. Text is in Spanish.
Gary Ritchison's page on Avian Reproduction: A fascinating page on reproduction of birds including eggs and the reproductive anatomy. The links are well worth following.
birdingart.com: Chris Lodge's new site featuring bird paintings.
Wildlife Online: Marc Baldwin's site featuring elasmobranchs (sharks, skates and rays) along with many mammals.
ARKive: a unique collection of thousands of videos, images and fact-files illustrating the world's species.

European Birds:

Birdguides: A great site for UK birds, especially the Rarities page.
The SOC: Lots of info about birds in Scotland and where to see them.
Dutch Birding Association: ditto for Holland
British Birds Magazine: Probably the oldest birding magazine in the UK if not the world.
Insh Marshes, Scotland: One of my favourite places. Site includes a good map.
Montrose Basin, Scotland: Another great site in Scotland.
The Wildlife Web: Bird news from the North East of Scotland.
Birds Poland: Information about birding in Poland.
UK Wildlife Helpline: Help with identification of wildlife in general.
Hungarian Birdwatching: Information on the birds and birding sites of Hungary.
Andalucian Guides: Information on birds and tours in the south of Spain. Greek Bird Photography: Some great photos from Panos Oikonomou.
Oiseaux.net: A very comprehensive site about French, European and World birds. Some of the content is only in French but much is in English and Dutch and the photos are excellent.

Birdsong

Brazilian Birdsong: Some recordings by Jacques Veillard
Doug Von Gausig: Comprehensive info about recording with many interesting samples.

Photography

Arthur Morris - Birds as Art: Apart from having a cool first name, this guy is one of the great photographers.
Stint Fan: Great photos of stints and other waders.
Marcelo Krause: Excellent photos from the Pantanal of Brazil..