This article doesn't mention some of the old standard examples:
Live Wire was a sculpture at Xerox Parc that twitched every time some number of network packets went through the office's router - you could get an intuitive feel for how much load was on the system.
There's an X Windows applet called "LavaPS" which shows all your unix processes as colored blobs in a lavalamp, sized by memory footprint, floating to the top by age. It gives you a quick impression of what your computer is doing - is one webserver process eating your whole core? Or are you getting forkbombed by thousands of little ones? Those look really different.
The definitive works on Calm Computing are a paper written by Mark Weiser (RIP) in 1996, "Designing Calm Technology" (which has unfortunately fallen off the internet) and an O'Reilly book called "Calm Technology" by Amber Case (the Cyborg Anthropologist)
Ideally we'd go calmer than a "status light" or "status tone" - those are still pretty active! What can you convey with the color of a Phillips Hue bulb that changes slowly?
Building requires the old gnome libraries (180mb on ubuntu)