Writing plugins - How to questions..

.ncb - internal MS Visual C++ file
.sln - project (solution) file for MS Visual C++

By the way, I might be interested in developing plugins, too, but I don't know where to start. I've done windows programming with Visual C++, but I've never written a .dll, so I'm not sure how that works.

It is not so difficult VC have templates so you can begin with them. Or take any sample project from TLB PDK and use it as template for your project. TLB uses COM technology for plugins. All you need is to implement ITlbButton interface. All methods of ITlbButton have good names (like OnLButtonDown). If you have any questions let me know I'll help you.

Do you think plugins can be written with Borland Delphi or Borland C++ Builder?

I guess Dephi and Borland C++ Builder can do this work too but I do not know these products well enough. So I cannot recommend you something