Data Planet--Great starting place for aggregate data (often longitudinal) on numerous economic, business, and social indicators.
_________________________
Bureau of Economic Analysis -- The BEA is an excellent source for macroeconomic data, with a focus on national accounts - GDP and its components such as income, consumption, investment, and government expenditure. It also provides much data on employment and compensation by industry. Data are available at the national, state, and local levels, in annual, quarterly and (in some cases) monthly increments. Some industry-level data and balance-of-payments data are also available. See http://www.bea.gov/itable/ for direct access to the data.
Bureau of Labor Statistics (BLS) -- The BLS contains much data on employment, wages, and prices, at both the national and sub-national levels. See http://www.bls.gov/bls/proghome.htm for a topical breakdown of the BLS' data holdings, and see http://www.bls.gov/guide/geography/ for a summary of data availability by level of geography. Be warned that the website is not always easy to navigate.
Census Bureau (U.S.) -- The Census Bureau gathers a dizzying amount of social, demographic, and economic information about the United States through various means. The Subject Index is a good starting point for browsing and searching through the Bureau's wealth of information. The Census Bureau's Business & Industry site provides a list of economic surveys conducted by the Census Bureau and the topics and geographic areas that each survey covers.
County Business Patterns (CBP) -- The County Business Patterns provide data for employment and establishments by industry, for counties, ZIP codes or Congressional districts. You can either query or download Business Patterns data.
Internal Revenue Service Tax Statistics -- This site from the IRS contains both reports and spreadsheets on various categories of taxes and income. Data are available for different types of tax forms and for taxes paid by individuals, by businesses, or by charitable organizations. Data are also available at various levels of geography, including states, counties, and ZIP codes. The statistics are generally published in various IRS publications and releases.
National Bureau of Economic Research -- In addition to its work on business cycles, the NBER has an eclectic data archive that covers topics such as cross-national technology adoption, manufacturing productivity, financial openness and exchange rate regimes, and economic policy uncertainty. Its data archive also covers topics such as labor markets, health economics, and population demographics.
Quandl -- Quandl is a data-aggregation site that provides access to roughly 7,000,000 indicators taken from multiple open, publicly-available sources such as government agencies (domestic and foreign) and international organizations. Note that Quandl classifies individual indicators as individual datasets. There are also tools to read datasets into various applications such as Excel and R and Stata
Statistical Abstract of the United States -- The Statistical Abstract contained a wealth of information on numerous socio-economic and demographic indicators for the United States. Historical data tables taken from the Statistical Abstract are available via the Internet Archive. While the Census Bureau has discontinued publication of the Statistical Abstract, the Abstract is now being published by ProQuest and is now available via Databases at Emory.
Economic Policy Uncertainty Index -- The Economic Policy Uncertainty Index project attempts to quantity economic uncertainty created by macroeconomic policy by coding media coverage, tax codes, and economic forecasts. The data are available in monthly increments
Federal Reserve Board: Data Releases -- The Board of Governors of the Federal Reserve system publishes a series of data releases on monetary and financial indicators on topics such as household finances, interest rates, exchange rates, industrial production, and monetary aggregates. Data from the various releases can be queried and downloaded via the Data Download Program.
Federal Reserve Economic Data (FREDII) -- FRED II includes time-series data for variables such as GDP, interest rates, exchange rates, consumer prices, and banking. There also also add-ins available to access and use FRED data in Excel, R, R (again), Stata, and Stata (again). Most of the data are from the 1950's onwards, though some series extend back prior to WWII.
Federal Reserve Bank of Chicago National Activity Index (CFNAI) -- The CFNAI is, to quote the website, a "weighted average of 85 existing monthly indicators of national economic activity ... The 85 economic indicators that are included in the CFNAI are drawn from four broad categories of data: production and income; employment, unemployment, and hours; personal consumption and housing; and sales, orders, and inventories." Data from a precursor to the CFNAI are available at this site. The CFNAI is one of many data releases produced by the Chicago Fed. Note that each of the district banks within the Federal Reserve system collects data and constructs indices on conditions within its district as well.
Federal Reserve Bank of New York Center for Microeconomic Data -- The FRBNY's Center for Microeconomic Data publishes quarterly reports and data both on household debt/credit broken down by source (e.g. student loans, auto loans, real estate) and on consumer expectations. Note that each of the district banks within the Federal Reserve system collects data and constructs indices on conditions within its district as well.
Federal Reserve Bank of Philadelphia - Research and Data -- The Philadelphia Fed conducts research and collects data on economic conditions within its district, including state-level indices on current economic conditions and leading economic indicators. Note that each of the district banks within the Federal Reserve system collects data and constructs indices on conditions within its district as well.