Spoiler warning: If you do not know yet, but want to read the Terry Pratchett novel Going Postal, please stop reading here. Spoilers below and in the links!

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The page XClacksOverhead.org lists public-facing websites broadcasting the X-Clacks-Overhead header aka Known websites carrying the signal.

I learned about this tribute to Terry Pratchett from a recent Golem.de article [German]: GNU Terry Pratchett: Eine Hommage für Eingeweihte an den Scheibenwelt-Erfinder

Citing Wikipedia:

Users of the social news site Reddit organised a tribute by which an HTTP header, "X-Clacks-Overhead: GNU Terry Pratchett", was added to web sites' responses, a reference to the Discworld novel Going Postal, in which "the clacks" (a semaphore system, used as Discworld's equivalent to a telegraph) are programmed to repeat the name of its creator's deceased son; the sentiment in the novel is that no one is ever forgotten as long as their name is still spoken.

A June 2015 web server survey reported that approximately 84,000 websites had been configured with the header.