Last Updated: 05/27/11
All of these sources contain free data!
Vector and Raster
Natural Earth - Vector and raster map data at 1:10m, 1:50m, and 1:110m scales. Very well presented. Try GIMP if you need to modify the TIFF files.
DIVA-GIS - Vector and raster data organized per country. Shapefiles for boundaries, water, roads, and railroads. Raster data for elevation, land cover, population, climate data. I have not used their raster data but it sure sounds interesting.
National Atlas of the United States of America - An incredible amount of data for the United States. Vector data is in shapefile and SDTS formats, and raster data is in GeoTIFF.
TIGER/Line Shapefiles - Features include roads, railroads, rivers, and legal and statistical geographic areas in the United States from the Census Bureau.
VDS Technologies - Links to several sources of vector data, many in the shapefile format.
NASA Visual Earth - Plenty of imagery, including their excellent Blue Marble Collection, with resolutions up to 500 meters.
VTP Whole-Earth Images - Links to many different imagery sources.
JHT's Planetary Pixel Emporium - Low and high resolution day, night, specular, bump, and cloud maps.
GIS icons - Icons for GIS applications available under a
Creative Commons Attribution-Share Alike 3.0 Unported license.
pinvoke - Quality icons available under a a Creative Commons Attribution 3.0 license.