Name: Dialog
Homepage: http://invisible-island.net/dialog/dialog.html
Open Source? Yes, GPLv2
Why this should be included: Minimal dependencies, used by many existing programs.
How many users: I suspect that although not many users will seek to integrate this into their own programs, it will be helpful to a large number of users trying to build existing, legacy programs.
Source: http://invisible-island.net/datafiles/release/dialog.tar.gz