The days of data shortage are gone. More and more DEM data sets are published under free licenses or as public domain. Through INSPIRE in Europe more and more will follow. The datasets below are not complete. This project will focus on free high resolution datasets with a spatial resolution of 2 meters and below on county/state scale. Have a look at the OpenDemSearcher map client on this website.
For low resolution DEMs here is a great map client.
Other sources, catalogues and lists for high resolution open DEM data: