This USGS topographical map was, I believe, last posted by Barnstormer66 HERE on the old GEC. This map is described as under "Mature Support" because it is no longer being updated, but I have found it to be an extremely useful tool. I have experienced few of the problems that he described, perhaps because I have a fast system and a very fast internet connection.
The only delay I have experienced is that one must zoom out until a map image appears, and then zoom in through the successive layers of resolution until the highest degree of resolution (or the degree you wish to use) appears. When moving to adjoining map segment, one usually needs to zoom out a bit, and then zoom back in.
I didn't find a download for it on a government site, but a direct download is the first thing that appears in a Google search. I am attaching that file.
There is another excellent resource I use which does not give a seamless topo like the NGS. Google Earth Library has a free download directly to Google Earth which overlays individual topo maps with all border information. I have used it many times to help to decode what I see in views of the US.
The lower border for a topo of the map of the center of the GE globe
Google Earth Library also has other useful free products and paid products.
And one more. I'm adding it last because it is slowest of all and at times the server does not respond. But it does have a small collection of gadgets: Modis Rapid Response System (my favorite), USGS Ortho 1 Meter (it used to be more useful, but for the most part GE has surpassed it) , USGS Topo Maps, and US Navy Cloud Top (pretty good world wide clouds and up to date). The original ooGEC thread is HERE.
You may want to put all of these in one folder as I have on my computer.