I used nationalatlas.gov for many years, downloading various shapefiles regarding water, roads, boundaries, environmental factors, and more. NationalAtlas.gov was a central public site that many GIS users in the USA relied upon regularly. Nationalatlas.gov now forwards to nationalmap.gov , which has a map viewer but does not provide nearly the level of functionality or shear volume of downloadable shapefiles. So, now that nationalatlas.gov is retired, where are the largest sources of public shapefiles that can be easily accessed?
I just checked my old National Atlas bookmark and was redirected not to the National Map Viewer, but to the National Map's Small Scale data page. There are 197 datasets available for download there, to my eyes it looks like it's the same data that was available at the old National Atlas page and they just moved it to the new URL. Are you not being forwarded to that site?
The main page states:
The National Map is now offering a collection of small-scale datasets that can be downloaded for free. Although the 1997-2014 Edition of the National Atlas of the United States was retired in September 2014, The National Map recognizes the importance of continuing to make a collection of the small-scale datasets, originally developed for the National Atlas, available to users.
It sounds like the layers that are there may not be updated going forward.
This site looks like it's got some unique data as well: http://www.naturalearthdata.com/