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."
IPUMS-International "is an effort to inventory, preserve, harmonize, and disseminate census microdata from around the world. The project has collected the world's largest archive of publicly available census samples."
NACIS and many volunteers support Natural Earth which is a "public domain map dataset ... featuring tightly integrated vector and raster data, with which you can make a variety of visually pleasing, well-crafted maps with GIS software."
The GeoSUR Portal provides an entry point to spatial data published by Latin American and Caribbean agencies.
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 Department of Agriculture GeoSpatial Data Gateway is an easy-to-use source for environmental and natural resources data, plus additional demographic layers.
For historical census data, visit the National Historical Geographic Information System, housed at the University of Minnesota.
For historical topographical maps, visit the National Map's Historical Topographic Map Collection, high-resolution scans of more than 178,000 historical topographic maps of the United States.
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.
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.
There a some interesting layers at the New York Ocean and Great Lakes Atlas: Data Viewer .
UNC GIS Data Finder, by themes (e.g., social, economic)
Rice University Fondren Library GIS Clearinghouses by State
The American Association of Geographers has lists of databases, maps & atlases, historical gazetteers and GIS.
OpenGeoPortal.org is a "collaboratively developed, open source, federated web application to rapidly discover, preview, and retrieve geospatial data from multiple repositories."
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.