It is too much of a hassel to create a link to ESRI legally. But they have good stuff.
Census Bureau - TIGER / Census data, boundary files, and online mapping. FREE DATA.
USGS Images - This link is particularly for data from the EROS Data Center in Sioux Falls, SD.
USGS Seamless - NED data @ 1/9 Arc Second, 1 Meter Orthoimagery - FREE DATA.
USGS NHD - National Hydrography Data and tools to work with them. FREE DATA.
TNGIS - Tennessee Spatial Data Server. Much data for and about Tennessee.
Geographic Datasets - Projects around the world and data for them... may not be helpful, but they
are interesting to explore.
GeoCommunity - Many types of data, much of which is FREE.
Indiana GIS Initiative - Jobs, Publications (infomation), Orthophotography...
Unioversity of Colorado at Boulder - Index of notes for Coordinate Systems, Data Conversion...