Spatial Data Resources
The Esri Maps and Data group provides access to online basemap and reference map services, map layers and documents, and tools to use in your GIS work.
David Rumsey Historical Map Collection
The collection focuses on rare 18th and 19th century North American and South American maps and other cartographic materials. Historic maps of the World,Europe,Asia, and Africa are also represented.
USGS repository for remotely sensed data. Free login will allow access to higher resolution data.
Earth Resources Observation and Science Center. Providing science and imagery to better understand our Earth; including U.S. topographic data and international DEMs.
Visualize data from numerous sources to identify trends that were before unseen. Map your data with real-time social data and the over 50,000 open-source data sets in GeoCommons.
Data.gov has integrated the Geospatial One-Stop (GOS) portal and catalog, and added some exciting improvements to both the user interface as well as the underlying infrastructure.
Global Spatial Data Infrastructure Association (GSDI)
Site with additional links for GIS and other data.
Free digital data and historic maps.
Tufts Geospatial Data Repository
Site allows you to search, discover, preview, interact with the map, and retrieve geospatial data from university repositories.
As one of the cornerstones of the U.S. Geological Survey's (USGS) National Geospatial Program, The National Map is a collaborative effort among the USGS and other Federal, State, and local partners to improve and deliver topographic information for the Nation.