Hammond's Flycatcher, Timber, Washington Co., Oregon on 18 May 2007 by Greg Gillson.
Online field guides are an incredible resource. Now, whatever field guide you buy for home use, you have others as close as your computer. Here are four covering North American birds.
eNature.com was the first online field guide, covering plants and animals in North America. The bird guide has 1-2 photos of each bird, audio voice recordings, and a written range description.
Sibley.eNature.com is basically a Sibley Guide to Birds online, with several drawings by David Sibley, audio voice recordings, and range map in North America.
WhatBird.com is the online inspiration for the iBird portable field guide application for the iPhone and iPod Touch. This guide has features to aid one to more easily find an unknown bird. There is much information here including a growing archive of photos from users. It displays a range map in North America.
AllAboutBirds.org from the Cornell Lab of Ornithology is clearly trying for the best online field guide. The range map shows both North and South American ranges for birds found in North America. Plus, it links to actual sightings in North America showing density as determined from the eBird data collection application. The site has audio voice recordings and there are over 12,000 bird photos (and growing) on the Flickr BirdShare site. It has a Birding Basics section with tutorials on introductory bird identification and photography.