If you see it, you probably know what it is because you installed a browser extension called something like “X-Clacks-Overhead” in memory of Terry Pratchett. If you click on it, it says “GNU Terry Pratchett”
What in the world?
The GNU Terry Pratchett plugin transmits an “X-Clacks-Overhead” header reading, “GNU Terry Pratchett” so that Terry’s name is whispered forevermore in the Internet’s “overhead”.
In Pratchett’s book “Going Postal”, workers who die in the line of duty have their names transmitted up and down the Discworld’s telegraph system as a tribute.
From Going Postal:
“We keep that name moving in the Overhead,” he said, and it seemed to Princess that the wind in the shutter arrays above her blew more forlornly, and the everlasting clicking of the shutters grew more urgent. “He’d never have wanted to go home. He was a real linesman. His name is in the code, in the wind in the rigging and the shutters. Haven’t you ever heard the saying: ‘A man’s not dead while his name is still spoken’?”
What does the GNU stand for?
In Going Postal, the G means that the message should be passed on, the N means “Not Logged”, and the U means that it should be turned around at the end of the line.
GNU Terry Pratchett