Postcardware
With software development, postcardware is an agreement where a program is free, but satisfied users are encouraged or required to send the author a postcard. To fulfill this obligation, a user can mail them a written postcard or send an electronic one.
The inclusion of postcardware license agreements serves as an acknowledgment for users and helps creators keep track of the software's usage. Above all, these licenses help add a human element to sharing technology.
Postcardware agreements are often based on trust, and, most of the time, licensing rules aren't enforced for practical reasons; postcardware doesn't include anything legally binding. In fact, some developers explicitly mention in the licensing agreement that sending a postcard is a choice, not an obligation.