Anyone heard of Pmail? It is a free email application written by a friend of mine who has done very well by just selling the documentation for the software.

The company that I work for sells a data recording and analysis system. We often refer to the data recorder as a "dongle" and it is that that we sell. Until a few years ago the software was pretty much considered "free". Then, mostly to emphasise to our customers that the software had value, we introduced a license that is keyed on the customer's name. This has the advantage that our software isn't keyed to a particular machine, but, because the customer's name is fairly obvious, makes it less likely that clients are going to rip off newer versions of our software and the added value plugins that we write.

Our licence code does not use heavy weight security, but it does manage to pack a bunch of constraint information into about 12-16 characters and is extensible to allow for constraints we've not thought of yet. If there is interest, I could ask if it were ok for me to work up a Perl variant of the code and publish it.