The US Bureau of Economic Analysis, part of the US Department of Commerce, provides high-quality economic statistics for the US economy, most prominantly the National Income and Product Accounts (NIPAs) which includes the estimation of Gross Domestic Product (GDP) and related measures.
The US Bureau of Labor Statistics, part of the US Department of Labor, provides economic statistics for the US economy, including consumer and producer price indices and employment/unemployment measures.
The US Census Bureau is most known for its US population surveys, but also surveys businesses and collects and reports economic indicators and foreign trade data.
The President's Office of Management and Budget's Historical Tables provide data on US federal government budget receipts, outlays, surpluses or deficits, Federal debt, and Federal employment over an extended time period, generally from 1940 or earlier to 2012 or 2016.
The US Bureau of the Public Debt is the US Treasury Department agency that borrows on behalf of the US government by selling debt. Track the US government's outstanding debt, even lend Uncle Sam money yourself.
The website of the Board of Governors of the Federal Reserve System offers data and information on monetary policy and banking regulation.
This third-party US National Debt Clock displays real-time projections from various US govertnment statistics, with a focus on government budget issues.
The CIA World Factbook provides information on the history, people, government, economy, geography, communications, transportation, military, and transnational issues for 266 world entities.
The University of Pennsylvania's Penn World Table 6.3 provides purchasing power parity and national income accounts converted to international prices for 188 countries for some or all of the years 1950-2007 (base year 2005).
The International Monetary Fund publishes a range of time series data on IMF lending, exchange rates and other economic and financial indicators.
The World Resources Institute's EarthTrends project provides searchable international data for a large number of environmental indicators.
Gapminder is a non-profit venture promoting sustainable global development and achievement of the United Nations Millennium Development Goals by increased use and understanding of statistics and other information about social, economic and environmental development at local, national and global levels.
The World Bank's Data Visualizer is similar to Gapminder's interactive graphical data analysis, using their own World Development Indicators database.