A project I have been working, rather slowly, is an irrigation controller, which uses a Pi Zero as the brains.
I am using a MCP23017 GPIO expander for my outputs, with ULN2803A Darlington transistor arrays providing the conversion from 3.3V to 5V (and allowing me to get enough current to drive my relays).
Given that the Pi Zero is so small compared to my relay board, it makes sense just to mount the zero on my board. To do this, I have used a stacking header, which will still allow me stack another board or connect a ribbon cable on top later (for expansion).
I have not finished the power supply or inputs yet. (Another couple of solder sessions to go, and still waiting for some Schottky diodes.)
The 3 pin header to the left of the Pi provides access to the serial console.
The little blue board which is connected vertically is a RTC. - I believe I cannot assume that the controller will boot with network connectivity. If it comes up with the wrong time, that could have some undesired effects on my irrigation times.
I guess, in a way you could say I have used the Pi Zero as a "Poor Man's Compute Module".