ESP8266
From Wildsong
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.