Resources to web sites that host large amounts of datasets both at the nationwide (United States) and global levels. Most of the data is available for free or for little cost.

Directions Data Center
This site offers free mapping data, in a variety of software formats.

EROS Data Center
The USGS EROS Data Center houses millions of aerial photographs of the United States, as well as images from several series of satellites covering the entire Earth.

Federal Communications Commission Datasets
Mostly MapInfo format vector datasets available for free download. Data examples include census data, FM Broadcast zones, and Guam mapping areas.

GIS Data Depot
Online inventory of more than a terrabyte of spatial databases spanning the entire globe. Free downloads or create customs CDs.

MapMart
Commercial data service. You can access free USGS DRGs.

National Atlas
National Atlas of the United States has nationwide datasets to view and download as ArcView shapefiles.

OpenStreetMap
Exportable geographic data for the entire globe.  Data is created and edited through crowdsourcing.  Click on the “export” tab to extract data.

USGS Geographic Data Download
Access to a wide variety of public domain data from USGS at a variety of scales. This page also contains information about converting SDTS and other formats to GIS compatible types.