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Dr. Peter D. Lanagan: Planetary Data

Mars Data

Find Mars data here.

Saturn-Titan Data

Huygen Descent Imager Spectral Spectrometer Data
Imaging data of Titan from the Huygens probe

Earth Data

Global Data

USGS Earth Explorer
Earth Explorer allows user to find images of much of the planet. Avaliable datasets include LANDSAT images, air photos, and dgital line graphics.
GTOPO30
GTOPO30 is a global digital elevation model sampled at 30 arcseconds per pixel.
USGS Geographic Data Download
Links to a variety of global datasets
USGS Seamless Data Distribution System
Allows downloading datasets covering customized region
GISDATA Map Studio
Provides updates on recently released geospatial datasets.
Earth Observing Satellite Data Gateway
EOS Data Gateway provides access to a variety of earth observing satellite data.

U.S. Geospatial Data

Geodata.gov
Clearinghouse of US geospatial data. The interface isn't exactly the most intuitive.
The National Map
A very useful online GIS viewer. I've found it useful for identifying datasets which cover regions of interest.
U.S. Census Bureau Maps
Geospatial data from the Census Bureau

Arizona Data

Arizona Geographic Information Council
Clearinghouse for geospatial data for Arizona
Arizona Map
The Arizona Map: An online GIS for Arizona
Arizona Regional Image Archive (ARIA)
Provides USGS digitized topographic maps, LANDSAT images, and digital elevation models for the state of Arizona
Arizona Land Resource Information System (ALRIS)
ALRIS provides GIS layers covering Arizona to the public for a nominal fee.
Arizona Electronic Atlas
Online maps for Arizona.
Pima County Technical Services GIS
A really useful government GIS for Pima County, Arizona. Well, it's really useful if you happen to live in Pima County.

Other Planetary Data

Planetary Data System
Clearinghouse for planetary data
National Space Science Data Center
NASA's permanent archive for space science mission data