GNU_Terry_Pratchett is a Firefox plugin designed to honor the legacy of Sir Terry Pratchett by displaying the message "GNU Terry Pratchett" across various websites that implement the X-Clacks-Overhead header. This tribute allows users to visually recognize and appreciate the message in a unique semaphore style, inspired by the Clacks board game. The plugin serves as a reminder of Pratchett's impact on literature and his contributions to fantasy storytelling.

The plugin is free to use and falls under the social networking category, enhancing the browsing experience for fans of Pratchett. It effectively combines nostalgia with modern web functionality, letting users engage with the tribute in an interactive manner. Overall, GNU_Terry_Pratchett is a thoughtful addition for those who wish to celebrate the author's legacy while navigating the web.