Holy cow, I wrote a book!
One item I left off the
list of special progids
Recall that Directory is the progid for file system folders
(a subset of Folder which represents all shell folders,
both file system and virtual).
Closely related is
which isn't really a progid, but it is a place where shell extensions
can register themselves.
Specifically, it's where context menu handlers can register if they
want to appear when the user right-clicks on a blank space
of a file system folder.