Postcardware is a term used in the software industry to describe applications that can be used for free, but the user is encouraged to send a postcard to the developer as a form of appreciation. There are several synonyms for this term, including donationware, where users are encouraged to donate a small amount of money to support the developer instead of sending a physical postcard. Similarly, there's nagware, where the user is frequently prompted to register the application after a certain number of uses. Shareware is another term used for applications that can be used for free but require payment for continued use. Overall, postcardware is a unique way for developers to receive recognition and encouragement for their work.