Since 2002 the amount and quality of public digital map data has been increasing marvelously. The United States has long had excellent coverage, but the comprehensive, worldwide coverage is increasing thanks to publicly-funded satellites, shuttle missions, and government agencies that process the data and provide it for free. Meanwhile the open-source community of dedicated hackers have maintained and improved software such as QGIS and GDAL to view, analyze, and edit this public data. We stand on the shoulders of giants…
BOUNDARIES, RIVERS, ROADS, RAILROADS: These types of data are in vector format as polygons and polylines. The best worldwide sources of data I have found are at the DIVA-GIS site, the similar-looking GADM site. The Mapability website facilitates access to vmap0 and vmap1 data. Country-specific data varies widely, often provided by national mapping agencies. More data continues to become available; the last time I edited this page (14 January 2012) I found the Russian GIS-Lab and the Natural Earth websites.
TOPOGRAPHY. The best topography is in “voxel” raster format, meaning that the data is in 3d pixels of longitude/latitude/altitude. There are two major sources of public, topographic raster data.
First, the National Oceanic and Atmospheric Association (NOAA) has issued a series of datasets of increasing accuracy, from ETOPO5 (5-minute) to ETOPO2 (2-minute) to ETOPO1 to GLOBE (30 arc-seconds of coverage, which means 120 pixels/degree).
The second is from NASA’s Shuttle Radar Topography Mission (SRTM). The SRTM data-set is ten times the GLOBE resolution, providing a 3 arc-second resolution which is about 90 x 90 meters at the equator. This is more useful for local maps, but would overwhelm your software for anything regional in scope!
SURFACE COLOR. For surface character there is the NASA’s Blue Marble series, in particular the massive 21,600 pixels x 10,800 pixels composite file which is 60 pixels/degree (1 arc-minute of coverage), and is 174 MB as an uncompressed TIFF file. This is a true-color image of the earth’s surface overlaid on a shadow-cast of topography. The true-color imagery comes from the MODIS sensor on the Terra satellite.