Probably the single most useful published collection of statistical information on virtually all aspects of life in the United States. And it usually directs you to the originally-published sources of data for more in-depth coverage.
Earlier editions also are available online from the Census Bureau. Printed copies (1878-2011) are available in the library's Reference collection.
Maintained by the Federal Reserve Bank of St. Louis, FRED provides 35,000 economic time series from 38 sources. FRED also provides an Excel add-in to access economic data directly from Excel.