There is a lot of GIS data available on the web, and below are some good places to start. Most data sets on these sites are free, although some are not.
BibleMap.org is a useful resource to visualize the historical locations mentioned throughout the Bible according chapters of the Bible. The motivation for developing the site was simple, create a free Bible atlas which harnesses Google maps.
NASA Earth Observation (NEO) provides a way to search and download satellite images of Earth (works with GoogleEarth) on topics of the ocean, atmostphere, energy, land, and life. It enables users to perform basic visual analysis by creating global mas to track regional and world-wide changes around the world.
The National Map Viewer allows you to interactively view The National Map data as a map, customize the view, and print a map. Map tools allow you to move around the map, zoom in and out, identify features, and perform other functions.
National Geologic Map Database includes maps and related data about: geology, hazards, earth resources, geophysics, geochemistry, geochronology, paleontology, and marine geology.
National Geographic MapMachine provides useful information on cartography, geographic names databases, government agencies, and map libraries.