GeoRef -- A comprehensive index to geology and related geosciences literature from journals, conference reports, government documents and more.
Greenfile -- Covers all aspects of human impact to the environment. Its collection of scholarly, government and general-interest titles includes content on global warming, green building, pollution, sustainable agriculture, renewable energy, recycling, and more.
Environmental Policy Collection -- From the University of North Texas, this digital library collection of documents "... contains a variety of open access resources that provide a balanced view on environmental issues and their potential consequences. The current focus of the collection is climate change."
Science.gov -- Searches for authoritative U.S. Government science information, including research and development results, from over 42 databases and over 2000 selected websites from 14 federal agencies.
OSTI.gov -- A portal to free, publicly available U.S. Department of Energy research and development results. Includes technical reports, bibliographic citations, journal articles, conference papers, books, multimedia, and data information sponsored by DOE from the 1940s to today.
TOXNET (Toxicology Data Network) -- A subset of Medline focusing on literature about toxicology, hazardous chemicals, environmental health, and toxic releases.
UN-HABITAT -- The United Nations Human Settlements Programme is the United Nations agency for human settlements and promotes socially and environmentally sustainable towns and cities. Find reports and publications on housing, planning and design, slum upgrading, and other urban development issues.