NASA and the University of Maryland maintain the Global Land Cover Facility which collects satellite data on land cover for the entire world.
The Lincoln Institute of Land Policy has produced the Atlas of Urban Expansion which provides downloadable maps and data related to the geographic and quantitative dimensions of urban expansion and its key attributes in cities the world over.
NASA and CIESEN/Columbia University maintain the World Data Center which "provides access to a wide range of global data, associated documentation, and visualization and analysis tools, and to the community of experts on global data."
A-to-Z Maps Online (accessed via Burke Library) includes a GIS Data tab with a number of sub-menus categorizing hundreds of GIS data sources.
The World Bank's Data and Research portal provides a wide variety of data, some of which is geographic.
The Department of the Interior maintains a Geospatial One-Stop Portal for federal, state and local geographic data.
The Federal Geographic Data Committee coordinates the sharing of geographic data through an online portal that searches metadata held within the NSDI Clearinghouse network.
The Census Bureau maintains the American Factfinder website, from which one can download census data in GIS-readable format.
For historical census data, visit the National Historical Geographic Information System, housed at the University of Minnesota.
DataPlace "is your free online source for housing and demographic data about your community, your region, and the nation."
New York State data is maintained by the New York State GIS Clearinghouse.
Another neat site, also maintainted by New York State, is NYS Orthos.
The Cornell University Geospatial Information Repository is also a clearinghouse for information about New York State.
The New York State Museum provides a nice collection of datasets for New York State.
UNC GIS Data Finder, by themes (e.g., social, economic)
The American Association of Geographers has lists of databases, maps & atlases, historical gazetteers and GIS.
The Geocommunity website bills itself as the "premier on-line resource for GIS and geospatial data."
New Urban Research, Inc., provides some data and shapefiles for free and some for a small charge at Primary Data Source.