British Petroleum Statistical Review of World Energy--The Statistical Review of World Energy is a very good source for international time-series data on energy production and consumption, with a collection of downloadable data here.
Carbon Dioxide Information Analysis Center (CIDAC)--The CDIAC is an excellent source for comprehensive data on climate change and related matters. Users may be especially interested in Trends Online, which includes much historical data on CO2 emissions as well as historical data on average temperatures.
Department of Energy Energy Information Administration (EIA)--The EIA is a very extensive source for data on energy consumption in the United States, with both annual and monthly data available. The EIA also has much international data available.
EarthTrends - the Environmental Information Portal--EarthTrends is sponsored by a collection of national and international agencies and contains statistics on a wide range of environmental and non-environmental topics. Each topic along the top of the EarthTrends site contains a searchable database of quantitative indicators that users can access.
Energy Information Administration (EIA) Environment Channel--The Department of Energy's environmental information site. Data on energy production, consumption, and environmental effects.
Environmental Sustainability Index (ESI)--The ESI is a project of Yale University and Columbia University that scores and ranks countries on their abilities to provide long-term environment protection. Data from ESI reports are available in both .pdf and Excel formats.
Environmental Treaties and Resources Indicators (ENTRI)--ENTRI is a searchable database of environmental treaties that allows users to create profiles of particular countries with regard to environmental treaties and agreements. Users can also search for treaties by topic of coverage and search through or browse full texts of treaties.
Paul Hensel's International Environmental Data Page--This site, which is hosted by Dr. Paul Hensel, contains links to data on a variety of environmental topics such as fisheries, energy, water disputes, and agricultural production.
United Nations Environmental Programme Data Portal--To quote the website, "[the] GEO Data Portal is the authoritative source for data sets used by UNEP and its partners in the Global Environment Outlook GEO) report and other integrated environment assessments. Its online database holds more than 400 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." Researchers are able to download the data in multiple formats.
World Bank Environment--This World Bank site contains links to various resources, including links to environmental issues in different regions and a collection of environmental data and statistics. The Bank also has a page devoted to environmental economics indicators with data on topics such as "green accounting."