Nessus provides and interesting example. The software itself is free, but you can buy knowledge modules.
So, Suppose a particular free package supports knowledge modules. These could be simple tables, scripts, whatever. A package like Nessus could even support its own programming language, perhaps even to the extent of LISP on Emacs. One can purchase them, and when purchasing them, they can buy a susbcription for updates. (For that matter, one could even purchase anti-virus signatures that destroy computers, but that is another story.)
So, the software is free, but the data isn't. Would scripts that run within a particular package constitute software that, by imnplication, would be free, or data that one would pay for?