I am not the biggest fan of shareware or other ways of limiting user rights when it comes to software, but if I had to pick one and call it my favorite, I’d go for the postcardware. Have a look at this good example:
Open source software is used in all projects we deliver. Laravel, Nginx, Ubuntu are just a few of the free pieces of software we use every single day. For this, we are very grateful.
When we feel we have solved a problem in a way that can help other developers, we release our code as open source software on GitHub.
A lot of our packages are postcardware —free to use if you send us a postcard.