I have literally hundreds of websites within a directory. They are all created with simple, basic HTML (PageSpinner). I would like to re-do them to become responsive. I see that CoffeeCup has three basic website creation/editing programs. I haven't found a clear explanation of the difference. How (which program) would you recommend that I get started?

RLM or RSD depending on how much external coding your willing to do. RLM is for developers that may be making websites for people and want a quick way to built a prototype frame work without images and other distractions. This allows them to get the major work approved before adding the fluff. RSD is for people that build websites but want a more complete tool that allows them to add images and other things directly within the app.
There's another app called the HTML editor and you may want that no matter which of the other apps you choose. You could hand code a responsive site with the html editor but it's a long tedious process. If you use RLM you may want the html editor to add that additional content after getting the frame work done. You may also want it for RSD to make fine adjustments or building in scripts for your pages. I have all three.

David, since you have been using Page Spinner, I reckon you are a Mac user. RSD and RLM can be used on a Mac, and if you have Parallels or Crossover installed, you can use HTML Editor. If not, Web Editor is for Mac. It hasn't been updated for a while, however, because CC are working on a new Mac editor.