Encyclopedias

Kirk-Othmer Encyclopedia of Chemical Technology
Valley  TP9 .K54 2004   (26 vol.)

Water Encyclopedia 
Valley TD351 .V36 1990

Handbooks

Environmental Contaminant Reference Databook
Valley
TD196.C45 P73 1995

Handbook of Environmental Data on Organic Chemicals
Valley
TD196.O73 V47 2001

Handbook of environmental engineering calculations
Valley
TD145 .H27 2000

Handbook of Environmental Fate and Exposure Data for Organic Chemicals
v. 1. Large production and priority pollutants. -- v. 2. Solvents. -- v. 3. Pesticides. -- v. 4. Solvents 2.
Valley
TD196.O73 M32 2006

Standard Handbook of Environmental Engineering
Valley
TD145 .S72 1999

Data.gov

Home of the U.S. Government's open data. Here you can search data within topic areas including agriculture, climate, ecosystems, energy, and more.

US EPA Search
The United States Environmental Protection Agency provides information and data sources on topics from air to pesticides to water. Source types can be narrowed to "Datasets" using the left hand search facets. 

World Bank Open Data
Search or browse by community or indicator.  Data is subject to a delay. 

Environmental Data Explorer
The Environmental Data Explorer is the authoritative source for data sets used by the United Nations Environment Programme and its partners in the Global Environment Outlook (GEO) report and other integrated environment assessments.  Its online database holds more than 500 different variables, as national, subregional, regional and global statistics or as geospatial data sets (maps), covering themes like Freshwater, Population, Forests, Emissions, Climate, Disasters, Health, and GDP.  Display them as maps, graphs, data tables, or download the data in different formats.

IUPAC Compendium of Chemical Terminology (former Gold Book)

NIST Chemistry WebBook

NIST Data Gateway:  Property Search

Physical Reference Data

PubChem  

Sigma Aldrich

Visual Elements Periodic Table