National Elevation Dataset (NED) was developed and is distributed by the United States Geological Society (USGS). It is seemless coverage over contiguous United States, Alaska and Hawaii available at different resolutions.
Here are some identified resolutions:
- 1/9 arc-second ( approximately 3 meters) – NED 3
- 1/3 arc-second ( approximately 10 meters) – NED 10
- One arc-second (approximately 30 meters) – NED 30
- Two arc-second (approximately 60 meters) – NED 60
- 7.5 arc-second (225 meters)
- 30 arc-second (1 kilometer)
- 15 arc-second (450 meters)
Here is a good explanation on how arc-second is measured.
NED datasets could be downloaded here.