A good online field guide—with images and information about 5500 different species—is at eNature.com, which is produced by the National Wildlife Federation. With sections on birds, mammals, reptiles, amphibians, fish, insects, spiders, seashells, wildflowers, and more; eNature is a good site for those of us who don’t have a complete shelf of field guides (I have almost worn out some of my Roger Tory Peterson guides).
One shortcoming: There are no range maps, showing the distributions of the species. Despite this, eNature is a fun and useful resource for identification of North American organisms.
Grace and Peace