A List of Data Sources
California Spatial Information Library
Contains Dems, Landsat images, Orthophotos, USGS Quads, etc. for the State of California
GLCF: Earth Science Data Interface
You can download Landsat images from all over the world here
USGS Topographic Maps for Arizona
Utah Division of Water Rights - Quadrangle Finder
Dems, orthophotos and USGS topographic maps for the State of Utah
From the MIT Center for Digital Geology Webpage