USGS National Map USGS site that contains seamless data for use in geospatial applications.
GIS Data Depot
Contains USGS 7.5 minute quad DEM maps and DRG (digital raster graphic).
MapMart Another great site for DEM data, aerial photography, satellite imagery, and much more
Offers free software to handle, display and merge electronic maps; also has conversion routines between NAD27 and NAD83 and other world coordinate systems.