Seamless Data Distribution System, Earth Resources Observation and Science (EROS)

This site (click here) offers free data downloads, user defined datasets and area, ability to acquirer data from different hosts, elevation point and profile tools, a tutorial and much more..(from the website).
Map Services Feature: For information on how to access seamless data using a map service, use the url http://gisdata.usgs.gov, in your GIS Applications
Web Services Feature: To access seamless data using a web service in your GIS Applications, see http://gisdata.usgs.gov for implementation details.