The Smithsonian Institution brings you North American Mammals, which is all well and good; a searchable database of mammals in North America, what’s not to like?
What makes this wicked cool, however, is that the site lets you make your own customized field guide.
You can click on a map, or input your latitude/longitude, and get a PDF-format field guide of all the mammals living in that area. Incredible!
It doesn’t provide the most detailed information or give the best illustrations, but with the ability to custom-build a field guide, what more can you really ask?