The free, GPL program qrencode is part of the
libqrencode project. For Windows users
it has been ported to Windows (I don't know how well it works on Windows)
Since GPL came up as being distinct from public domain (which it indeed is), I think it should be noted that the GPL imposes *no* restrictions or obligations on
users of GPLed software. You can use it however you want, including commercially.
What the GPL does is restrict how you can
distribute the software - if you distribute GPLed software (including if you incorporate GPLed code into your own program) then you must also provide a means for whoever you distribute the software to to also be able to get the source code for the software.
In summary - GPL restrictions only come into play when distributing GPLed software.
(note: there are uncommon exceptions when the output of a GPLed software contains text, images or audio that is embedded in the software itself)