My memory tells me that that Sinclair ZX81 could easily control a mains socket and therefore allow a user to do things like control the watering of pot plants without needing to get a soldering iron out or learn hacking skills.
Unfortunately, my memory seems to have been wrong, because I looked in the ZX81 manual and it contains nothing about this function. So I must be thinking of another very early model of home computer.
Anyway, what I need is a hardware device that I can plug into my Windows laptop, preferably into one of its USB sockets, and then wire a mains socket into it that the driver software for the device will be able to turn on and off.
I am aware of "smart plugs", but this is not what I am asking about.