Useful Geologic Links

National Geologic Map Database (NGMDB) at the USGS: the best place to start looking for geologic maps.
National Spatial Data Clearinghouse (NSDC): a search engine with access to a HUGE collection of maps.
National Digital Atlas: a good way to quickly make customized thematic maps.
The Association of American State Geologists: good place to get detailed info on a state-by-state basis.
Digital Chart of the World at Penn State
Color Landform Atlas of the United States: a terrific way to get topographic and satellite images of the states (mostly GIFs). Courtesy of Ray Sterner.
GLOBE (Global Land One-kilometer Base Elevation) digital elevation (DEM) data for land areas: covers the entire planet! 30 arc-sec (about 1km) gridded elevation data.
GTOPO30: another set of global land elevation data at 30 arc-sec (~1 km) resolution.
ETOPO5: global dataset of topography (land) and bathymetry at 5 arc-min (about 10 km) resolution.
Want a simple basemap quick? Try the easy-as-pie, GMT-based web utility Map-It.
GMT: set of Unix tools for geographic data manipulation and mapping from Paul Wessel and Walter Smith.
GRASS: open source GIS
Microdem by Peter Guth of the U.S. Naval Academy.
Earth Images Catalog
Mapquest
Expedia
Marli Miller Image Database: Thousands of geologic images in a searchable database + maps of national parks and other useful documents!