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..

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.

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.

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..