Here are some of the internet bird references that I find most useful. Obviously there are lots more, and this is just a beginning. Feel free to suggest additions in the comments.
Birds of North America online – the best single reference source (subscription required)
SORA (Searchable Ornithological Research Archive) – Incredible searchable and downloadable library of most of the major ornithological journals. Most journals provide content at least five years old, and do not offer newer material for free.
The AOU Checklist of North American Birds – the last word on names and taxonomy
The IOC Nomenclature – an international perspective on bird names
Tree of Life – the “family tree” of birds of the world, giving a helpful visual representation of relationships
History of North American bird names, 1886 to present – all the AOU checklists in table form
Search the archives of Frontiers of Identification listserve – expert discussions of challenging ID issues since 1995
Wing Image collection at the University of Puget Sound – an incredibly useful resource if you need to study wing patterns
Ridgway’s Birds of North and Middle America on Google Books – a classic and still useful
Identification, age, and sex of ducks from wing patterns – The USFWS document
Macauley Library of Natural Sounds – the biggest and best collection of natural sounds (and videos), all searchable and accessible at the website
Xeno Canto – a huge library of bird sounds from all around the world, with a great interface
Earbirding – Nathan Pieplow’s excellent blog on bird sounds and related issues
Odds and ends
Guide to age of nestling passerines pdf here