USGS Seamless Data Warehouse USGS site that archives free NED (National Elevation Dataset) for use in geospatial applications.
GeoDATA US Goverment site for federal, state and local geographic data.
GIS Data Depot
Contains the above maps as well as some maps no longer available at the USGS site, such as 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.