Online References and Field Guides

Audubon Society Online Field Guides
Excellent source of information and comprehensive. The use of scientific language may not be the best.
Glossary of Botanical Terms
Many definitions and explanations of basic botanical words.
Hinterland Who's Who
May not contain all species we want but excellent, detailed, and accurate information about each listed species.
National Wildlife Federation Online Field Guides
Comprehensive guides for North America.  This is a very useful resource covering both the plant and animal kingdoms.
Online Data Sets
A wide array of data sources (mostly from the USA)
Skull Identification
Excellent way to identify that old skull you found on the trail.