Over a year ago, I backed the gen 1 Onion Omega. It’s a tiny device (28.2mm x 42mm) with a 400MHz processor on board, built in WiFi, running Linux (OpenWRT + BusyBox). It’s designed to be an IoT device, but is basically a great device for anything that a Raspberry Pi is too big for. I’ve used it build a few silly prototype controllers but it will soon end up controlling a dehumidifier in my shed. This can be easily achieved using a good amount of add-ons via GPIO that you can buy from Onion and also from other suppliers.
I raise this because the gen 2 model is on Kickstarter and you can pick one up for as little as $5 (https://www.kickstarter.com/projects/onion/omega2-5-iot-computer-with-wi-fi-powered-by-linux).