I've seen various people using a "sonoff" device which is basically an ESP8266 driving a relay, but you can flash it with any arduino code, this chap has done various MQTT, and API hack stuff with them
Thanks for the link. If I had to do it myself, I was thinking of doing it with an ESP8266 (I have one sitting in a drawer in the workshop, together with an Arduino Mini and some relays). I'd just like a quick fix with an over the counter package for this...!
Do you need wifi? A lot of them are low-power RF (eg 433MHz), with a very simple transmitter (using air gap isolation). These ones have a Raspberry Pi module and library:
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