I hear there's a new WiFi microcontroller that is cheaper than ESP8266's "unbeatable" low price tag.
It is the Air602, a 2.4GHz Wi-Fi module based on Winner Micro W600 system-on-chip.
Buying an Air602
Currently, Seeed Studio is the only seller for this module.
It comes as a bare module for $1.90, or a development board for $2.90.
The bare module lacks an antenna and requires surface mount soldering that I'm not comfortable with, so I purchased the development board version that comes with antenna, power regulator, and USB-UART chip.
My order was shipped from Hong Kong, and it took about two months for the order to arrive United States.