In the always-online sci-fi video game Destiny, there is a secretive merchant who appears on an irregular yet calculable schedule: Xur. He sells rare and exotic items you don’t want to miss, yet he is easy to miss.
This was a quick weekend project with friends to create a utility website and Twitter bot to notify players of his presence.
It became very popular very quickly, and although I did not stay on the project, it has continued to grow with Destiny and Destiny 2.