I recently stumbled in to this question while doing research. I don't have a complete simple answer either, but there are some other options not listed in the answers so far:
- Google Elevation API: 25,000 requests/day, limited to Google Maps applications
- Lat/Lon to Elevation: 2 points / second
- GPSVisualizer: no published speed, but not intended for general DEM use
- Shuttle Radar Topography: alternative to the NED, 7 gigabytes for the US.
