ESP8266

2.4 GHz WiFi microcontroller module

Originally people used these as a sort of modem and serial cable without the wire. They sent data over WiFi via Hayes modem AT commands.

Then someone figured out you did not need to connect a separate microcontroller. There is already one in the module.

I have this one:

ESP8266MOD Ai-Thinker ESP-12E

Can be programmed with the Arduino SDK, see this very helpful Instructable to get started.

The story continues in this article on putting a web server on the tiny little card.