There’s the Sonoff ZBMINIR2 device. You install it between a physical switch and the wiring, and then tuck it into the electrical box. It has three different modes:

1. As a relay, so that you can control the light either with the physical switch or by ZigBee. Works just like normal when the server is down.
2. Detached mode, so toggling the physical switch sends a ZigBee message to trigger an action on the server, and the server controls the light.
3. Like 2, but the light socket is always powered so it supports a smart bulb.

I have a couple, and they’re great. They just don’t support dimming.