References: Spatial Data

Map Data: There are several sources of map data online, including:

CIA's World Data Bank II
U.S. Census Bureau TIGER Products
United States Geological Survey (USGS) Data

GPS Data: There are several sources of GPS data and tools for working with GPS data online, including:

GPS Timing Data
GPS Status Information

Catalogues of Spatial Data: There are several catalogues of spatial data online, including:

Digital Elevation Data Catalogue
The Ohio State University Spatial Data Catalog
Stanford University Library GIS Data Catalog
United States Geological Survey (USGS) Catalog
University of Virginia Geostat Catalog