High-Level Environmental Data Source and Clearinghouses [also see Metadata]

National Biological Information Infrastructure (NBII), a broad initiative of the US Government
National Environmental Data Index (NEDI) I NEDI Search Page | Biodiversity, Systematics, and Collections

Environmental Fate Data Base for assessing chemical exposure and contamination (supported by the U.S. EPA, Procter & Gamble, and Du Pont)
Databases listed by Yu-Fai Leung's Natural Resources Research Information Pages
San Francisco Estuary Institute San Francisco Bay water quality
Digital Library Project at the University of California at Berkeley:
      Botanical Data | Geographical Data | Zoological Data
Scorecard.org, an Environmental Defense Fund site that tracks environmental pollutants in the United States.
Master Environmental Library US Government data (primarily geophysical and weather related)
National Environmental Satellite, Data, and Information Service (NESDIS) of the NOAA (National Oceanic and Atmospheric Administration of the United States )

Web Mining Prototype at the DOE's Environmental Science Network

Global Biodiversity Informatics Facility (GBIF), a new international initiative
Global Network of Environment and Technology (GNET)
European Environment Information and Observation NETwork (EIONET)
NRM_Changelinks, resources for sustainable natural resource management, Massey University, New Zealand
Center for International Earth Science Information Network (CIESIN) at Columbia University
Information Center for the Environment (ICE) at the University of California at Davis
United States Geological Survey (USGS)
      USGS Water Resources page