The NCSES, part of the federal National Science Foundation, provides datasets and data-driven studies on a range of science and engineering issues.

The federal government's official portal for data sources. These include the departments of Energy and Transportation, the Consumer Product Safety Commission and NASA.

A searchable database of research data.

A searchable "global registry of research data repositories that covers research data repositories from different academic disciplines" (information from re3data).

Includes extensive data on energy sources, water sources, and geological characteristics and behavior.

The USGS's NWIS database provides access to data from more than one million water sites.

The EPA's repository for water monitoring data.

The EPA's database for water quality assessments of sites with impaired quality.

The EPA's tool for identifying watersheds in an area and access their WQX monitoring data and WATERS water quality assessment report.

The DOE provides multiple repositories for data and other information. Hosted by the DOE's Office of Scientific and Technical Information (OSTI).

Provides extensive data on US and international energy production, usage, and prices. Most data is organized by fuel type.

Contains worldwide data on energy production, consumption, and trade.

The EPA's resource for data on emissions generated within the electric power industry.

Provides datasets on fuel economy, vehicle emissions testing and other metrics.

Provides data on alternative and renewable fuels, new vehicle technologies, and other measures designed to support fuel economy.

Provides detailed statistics on the movement of people and cargo, infrastructure, investment, and more. BTS also links to transportation data on the sites of other federal agencies.

Includes US Census data on commuting patterns.

A list of municipal and metropolitan sources for public transit data. These sources typically contain data on routes and schedules; and ridership.

NADA, one of the leading industry groups for the automotive industry, publishes data and annual reports covering sales, services, inventories, and more.