The website I once used "terraserver-usa.com" no longers exists (as of 2023).
At the time I worked on it, Terraserver was involved in some way with Microsoft, and looked like it might have been an accessible alternative to Google maps.
My idea was two fold. One was that I liked the idea of overlaying satellite imagery with the TOPO! series maps I am working with and then having a transparency slider (or an ability to blink from one to the other with a single keystroke. I also was keen on the idea of using a publicly available map database in order that the Gtopo program might be useful to people without the map files on disk like I have.
It would be interesting to pursue this using Google Earth imagery, rescaling it so that it would overlay the gtopo map displays.
Gtopo / tom@mmto.org