X-Clacks-Overhead is a Firefox extension that pays homage to the late author Sir Terry Pratchett by displaying the message "GNU Terry Pratchett" on websites that utilize the associated HTTP header. This extension enhances the browsing experience by detecting the presence of the 'X-Clacks-Overhead' meta tag in web pages, ensuring that users can appreciate this tribute wherever it appears. It is a fork of the original GNU_Terry_Pratchett extension, now optimized for better functionality with meta tag detection.

The extension features a visual representation of the message in a semaphore style, inspired by the board game 'Clacks' by Backspindle Games. With its straightforward installation and free license, X-Clacks-Overhead serves as a meaningful tool for fans of Pratchett's work, allowing them to connect with his legacy while browsing the web.