American FactFinder provides access to data about the United States, Puerto Rico and the Island Areas. The data in American FactFinder come from several censuses and surveys. Check out the ERC's guide to using this database (PDF file).
This is a great source for empirical research and data on subsidized housing, land use, real estate development and and other useful New York City neighborhood and housing data. From NYU's Furman Center for Real Estate and Urban Planning.
The National Historical Geographic Information System (NHGIS) provides population, housing, agricultural, and economic data, along with GIS-compatible boundary files, for geographic units in the United States from 1790 to the present.
The Statistical Abstract of the United States is an authoritative and comprehensive summary of statistics on the social, political, and economic conditions of the United States. Once provided free from the U.S. Census Bureau, now provided by ProQuest.
Passport from Euromonitor International is a market research tool that monitors industry and product trends in countries around the world. They also have selected city reports. You can search for these by the name of the city or browse available reports - click on "Economies" in the top navigation bar and then "Cities".
UN Data provides statistical time series for countries from around the world covering a wide range of economic and socio-demographic topics. It includes descriptions of the international sources and definitions used in compiling the data.
World Council on City Data Open Data Portal is based on the first international standard on city data — ISO 37120. The WCCD Open City Data Portal allows you to explore, monitor, and compare member cities on up to 100 service performance and quality of life indicators.
Housed at the Institute for Quantitative Social Science (IQSS) at Harvard, this claims to host the world's largest collection of social science research. It holds data from many researchers and institutions in addition to Harvard.
ICPSR is one of the largest repositories for social science data in the world. Among the many topics covered, you will find statistics on crime, education, demographics, elections, religion, and much more.