IDENTIFYING SPECIES — WEBSITES:

General:
Exploring Nature Educational Resource (registration required)

Wildflowers & Plants:
USDA Plants Database
eNature Wildflower Field Guides

Amphibians:
USGS Amphibian Species and Identification Guide

Turtles:
Nearctica Natural History -- Turtles

Snakes:
University of Pittsburgh -- Snakes of North America

Reptiles & Amphibians In General:
eNature Reptile & Amphibian Field Guides

Insects:
BugBios: Entomological Database of Very Cool Bugs
BugGuide.net
eNature Insect & Spider Field Guides

Birds:
Cornell Lab of Ornithology: All About Birds
eNature Bird Field Guides

Butterflies:
Butterflieds and Moths of North America
eNature Butterflies Field Guide

Trees:
National Arbor Day Foundation
eNature Trees Field Guide

Mammals:
eNature Mammals Field Guide

FOR IDENTIFYING SPECIES ON THE TRAIL - BOOKS:

Wildflowers & Plants:
Newcomb's Wildflower Guide
Newcomb, Lawrence. Newcomb's Wildflower Guide, Little Brown and Company, 1977

Reptiles and Amphibians In General:
A Field Guide to Reptiles & Amphibians
Conant, Roger. A Field Guide to Reptiles & Amphibians. Houghton Mifflin 1998

Insects:
A Field Guide to Insects
Borror, Donald J. and White, Richard E. A Field Guide to Insects. Houghton Mifflin, 1970

Birds:
Birds of North America
Robbins,Chandler S., Birds of North America, Golden Field Guides, St Martin Press, 2001

Butterflies:
National Audubon Society Field Guide to North American Butterflies
National Audubon Society Field Guide to North American Butterflies. Knopf; Chanticleer Press Ed edition 1981

Trees:
Trees of North America
Brockman, Frank C. Trees of North America, Golden Press, 1979

Mammals:
A Field Guide to Mammals
William H. Burt and Richard P. Grossenheider. A Field Guide to the Mammals of all North American, Houghton Mifflin, 1976