rwho
The BSD command that showed who was logged in on every machine on the local network — because every machine announced it.
rwho shipped with 4.2BSD in 1983. The rwhod daemon broadcast a UDP packet every three minutes listing the machine's logged-in users, load and uptime, and collected its neighbours' broadcasts in return. rwho listed the users; its sibling ruptime showed the machines. Handy in a terminal room, where you wanted the least loaded box or to find out where a colleague was before you talked to them.
With hundreds of machines on one segment the broadcast traffic became noticeable, and having every computer tell everyone who was logged in became a security problem. rwhod was switched off by default and joined finger on the pile of services nobody wanted to run anymore.