Technically the ESP8266 is a modular wifi controller, isn't it? That is, it's one of those gadgets that's supposed to sit on a daughterboard and provide wifi services with embedded TCP/IP stack to another microprocessor. (I hear.) It just so happens that it's own processor is powerful enough and flexible enough to be useful as a microprocessor in its own right.

I wonder what other common bits of electronics might have useful amounts of processing power...