GNU Terry Pratchett. Show user when 'Clacks-Overhead' appears in HTTP headers.
NOTE: See paragraph 1 at the bottom of this description regarding required permissions.
To quote the twitter account of the great man himself:
"AT LAST, SIR TERRY, WE MUST WALK TOGETHER."
Terry took Death's arm and followed him through the doors and on to the black desert under the endless night.
See here: http://www.gnuterrypratchett.com/
-Peter Bell (original author)
-Charlie Wheeler-Robinson (bug hunting, refactoring, improving in early versions)
-Jimmy Nyström (code)
-Melanie Wilberforce (icons)
-Rob Grundy (who made the Firefox extension) for sending me the clacks icons used in the animation
-Backspindle Games for producing the clacks icons in the animation
-Sap1ens, jwakely, and Amy Atha-Nicholls (generously offering new icons)
-yousuf811 on Github (squashing bugs)
-Jarek Glowacki for some CSS help
-See GitHub issues and pull requests for others
A big thank you to all of the lovely people who sent new icons (listed above)!
The Chrome Developer Dashboard sucks and doesn't notify me of feedback - I try to keep an eye on it but if I don't reply, email me (firstname.lastname@example.org). I take no responsibility for any impact on browser performance as a result of this extension, though I believe it should be negligible. I use this extension myself and am completely open to suggestions or improvements.
I don't know anything about licences so I'm including one offered by Github just in case because it seems pretty reasonable. This is a simple extension and a tribute to Terry so I'm not really worried what you do with it.
Note 1: There are people complaining about the permissions required. That's fair enough, I would be too. The latest updates need 'webNavigation' permissions for reading the HTML meta tags (a feature which a lot of people were asking for). That is what requires access to web page content. The original permissions required by older versions were for webRequest, which is necessary for the basic HTTP header interception. As far as I know, there isn't a better way of doing it. If you are still concerned I suggest downloading it directly from Github (link above) so you know exactly what you are getting, you just won't get automatic updates. I wish there was a way to transparently publish directly from the public version on Github but as far as I know, the only way is to upload zip files myself. There is no way I can guarantee I haven't changed anything in between.