enature.com from the National Wildlife Federation features:

Online Fieldguides for plants, birds, mamals, fish, insects and more
Article Series like Sky guide, Birding Focus, Gardening, and On the Wildside
Local Nature features Zip guides (species locator), Parkfinder, Gardening, Backyard Wildlife and Wildlife list section

While your there, don’t miss the Fun Stuff section for ecards and screensavers.