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."
You can download country boundaries from Global Administrative Areas in several different formats.
ESRI hosts Open Data, where people can easily find and download data in a variety of formats.
Some global climate and ecological data is available from DIVA-GIS.
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 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.
The Center for Population Economics has created a historical GIS dataset, exploring and analyzing the urban health environments of seven major US cities—Baltimore, Boston, Brooklyn, Chicago, Cincinnati, Manhattan, and Philadelphia—from 1830 through 1930.
For all topographical maps and data layers, visit the National Map, maintained by the USGS.
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 NYS Office of Planning & Development has a new Geographic Information Gateway featuring not only data sets and maps but stories of how this data is being used, for example, in climate change and shorelines.
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)
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."
The Thematic Mapping Engine allows you to create a file of UN data which can be viewed in Google Earth.
Also see the GEHacks Download page for KMZ files of cities, events and image overlays that can be displayed in Google Earth.