The tree of life
Biodiversity and Biological Collections Web Server Caution--a little out-of-date but has some good links.
Flowering Plant Gateway allows comparison of four systems of plant classification.
Animal Diversity Web
Zoological Record Internet Resource Guide for Zoology.
Government agencies support related information sources. The U.S. Fish & Wildlife Service's Fish and Wildlife Species (Wildlife Fact Sheets) and the U.S. Geological Survey's North American Reporting Center for Amphibian Malformations are two examples.
Flora of North America
Catalogue of New World Grasses
Kew Record of Taxonomic Literature
International Plant Names Index now also includes part of the Gray Herbarium's Index of new world plants
Checklist of Online Vegetation and Plant Distribution Maps