The Shelly 1 is a small device with a WiFi-capable microcontroller and a relay, wrapped up in a neat little box with appropriate gubbins to wire directly to the mains. Typical use case is that you bury it in a junction box or light fitting, and wire it up in such a way that it controls the power to the light. So far, so smartbulb. The clever bit is that you connect the existing switch wire to the Shelly's input terminal, so it knows what the switch is doing, but the Shelly stays powered up at all times.
You can then program it to do whatever clever stuff you might reasonably program it to do. Typically, if you frob the light on or off with your phone or a timer program or whatever, changing the state of the lightswitch will frob it the other way (ie. it behaves as if the switch is now part of a double-switch circuit with the smartness, which is intuitive even for people who don't know it's there).
They have a whole range of similar devices (more advanced ones have multiple outputs or inputs, energy meters, and so on), and are one of the few internet-of-things products I really rate. Not least because they don't lock you into using their own web service to communicate with them. Indeed, they're designed in such a way that it's reasonably easy to write your own firmware for them if you want to. (I have a Shelly Plus 1 hidden behind the landlord's crappy timer that I've written my own firmware for operating as a smart thermostat.)
