For descriptions of the various datasets that can be obtained from the US Geological Survey please visit their site at:

http://edc.usgs.gov/glis/hyper/guide/usgs_dem

DEM files can be found at:

USGS Main Digital Elevation Data Distribution Center

California State University, Northridge 30 Meter DEMs  (United States)

Canadian Digital Elevation Data

Continental USA 500 meter resolution DEM

Global Land One-km Base Elevation (GLOBE) Project - National Geophysical Data Center

Mount Saint Helens Sample - USGS

NOAA Estuaries of the USA bathymetry

US Geological Survey GTOPO30 DEMs

US Geological Survey One Degree DEMs (Entire United States)

US Geological Survey 1:250,000 DEMs

US Geological Survey 1:24,000 DEMs - SDTS format only

A vast collection of DEM & Digital Cartography links can be found at:

University of Iowa Center for Global and Regional Environmental Research

University of Edinburgh - Bruce Gittings' Digital Elevation Data Catalog