Comment by techwolf

I picked up a few ESP01 boards from Aliexpress recently, they're less than $1.50 each. They only have two GPIO pins accessible (and one needs a pull-up resistor to make the ESP8266 boot properly) but for toggling a relay or reading a simple sensor over wifi they're hard to beat. They're even powerful enough to run a basic HTTP server to handle a light web interface for the GPIO.