GADM is a spatial database of the location of the world's administrative areas (or adminstrative boundaries) for use in GIS and similar software. Administrative areas in this database are countries and lower level subdivisions such as states, provinces, departments, etc.
The GLCF is based at the University of Maryland and is a center for land cover science with a focus on research using remotely sensed satellite data and products to access land cover change for local to global systems. It provides both raster and shape files.
This is the resource website of the Map and Geospatial Information Roundtable of the American Library Association. Much of its content is aimed specifically at map and geospatial librarians, but it also has sections devoted to geospatial data and tools.
This is a public domain map dataset available at 1:10m, 1:50m, and 1:110 million scales from North American Cartographic Information Society. It features tightly integrated vector and raster data of physical and cultural features.
SEDAC is part of the Earth Observing System Data and Information System (EOSDIS) of NASA, and it is hosted by Columbia Univeristy. SEDAC focuses on human interactions in the environment. Its mission is to develop and operate applications that support the integration of socioeconomic and Earth science data and to serve as an "Information Gateway" between the Earth and social sciences.
The Environmental Data Explorer is the authoritative source for data sets used by UNEP and its partners in the Global Environment Outlook (GEO) report and other integrated environment assessments. Its online database holds more than 500 different variables, as national, subregional, regional and global statistics or as geospatial data sets.
WorldClim is a set of global climate layers (climate grids) with a spatial resolution of about 1 square kilometer. The data can be used for mapping and spatial modeling in a GIS or with other computer programs.
The "geo" portion of the US government's data.gov initiative. This is a good starting point if you want government geospatial data, and you don't know which agency or department might be the source of such data.
The National Historical Geographic Information System (NHGIS) provides, free of charge, aggregate census data and GIS-compatible boundary files for the United States between 1790 and 2011. Users must register to download data, but there is no fee for registration nor any spam.