This article referenced the ESP8266 (specifically NodeMCU configuration) as a cheap alternative for WiFi IOT projects that don't warrant the Pi's complexity. I've played with this chip and was pleasantly surprised how easy it was to get it working. The bare chips can be had for a couple of dollars, or $15 for a development board which supports USB flashing.

Don't cheap out on the dev board, the cheapest ones use a shitty USB-serial chip and it was a headache to get the drivers working. The adafruit one worked out of the box.