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

Australian Digital Elevation Model

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

GTOPO30 World DEMs - Free

Mount Saint Helens Sample - USGS

NASA Mars Viking Orbiter Digital Terrain Models (Entire Surface of Mars)

NOAA Estuaries of the USA bathymetry

San Francisco Bay Area Regional Database 30 Meter DEMs  (United States)

Surface Maps from US Census TIGER Map Server (Entire United States)

United Kingdom DEM data

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

Washington State Geospatial Data Archive

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

Alpine Map Guild Resources

University of Iowa Center for Global and Regional Environmental Research

University of Edinburgh - Bruce Gittings' Digital Elevation Data Catalog