rusers
A Unix command that asks the machines on the local network who is logged in, using Sun RPC and the rpc.rusersd daemon.
Where rwho had every machine constantly broadcasting its status, Sun built a variant that only asked when someone wanted to know. rusers sent an RPC call as a broadcast, and every machine running rpc.rusersd replied with its list of logged-in users. It shipped with SunOS and spread to other Unix flavours during the 1980s, in the same family as finger and talk.
On a campus network it was a handy way to find a colleague or an idle machine. It was also a free list of valid usernames for anyone keen on guessing passwords, and rpc.rusersd ended up among the services hardening guides told you to switch off first.