FedStats collects in one place all the statistical data produced by federal agencies, including the Census, the Bureau of Labor Statistics, the Bureau of Economic Analysis, the Economic Research Service, Statistics of Income, the Small Business Administration, and many, many others. Constantly updated, this is a great place to start if you want to make sure you're getting statistical info on a subject from every possible federal agency that might have something to do with it.
"The Business Dynamics Statistics (BDS) provides annual measures of business dynamics (such as job creation and destruction, establishment births and deaths, and firm startups and shutdowns) for the economy and aggregated by establishment and firm characteristics." Current coverage: 1976-2012.
"The Economic Census is the U.S. Government's official five-year measure of American business and the economy. It is conducted by the U.S. Census Bureau, and response is required by law." Last done in 2012, next to be done in 2017. There is a LOT of information here, and can be searched by Industry, Economic Topic, Geography, and Demographic.
"[T]he authoritative and comprehensive summary of statistics on the social, political, and economic organization of the United States." The Statistical Abstract was compiled by the federal government from 1878 until 2012, when its budget was cut entirely. It is now published by the database company ProQuest, but is behind a paywall. The historical Abstracts, however, are still quite valuable when it comes to figuring out what the country was like at a certain point in time. The full volumes are available for download.
Statistics about American businesses that reflect "the number of firms, number of establishments, employment, and annual payroll for most U.S. business establishments," analyzed by geographical area, industry, and business size. Based on the current Census data, conducted in years ending in -2 and -7.
Statista culls its data from a number of market research companies, government filings, demographic research, trade groups, and others both domestic and international, and analyzes that data. Some is available without registration but download requires at least a basic account, while a fair amount is behind a paywall.
Partner to Statista but no registration or fee required. Statistic Brain's statistical analysis makes its sources obvious (as every interpretation of data should) and draws from mostly domestic information, covering Marketing, Financial, Industry, Company, Demographic, and Geographic types of stats, and several topics such as Education, Health, Crime, Food, and more.