This is a project for ESP8266 for low power sensor applications. The chip powers on once every 1 minute to measure it's own power supply, and sends the data to the server, once every 20 minutes. Of course, you can configure the chip to do other things such as measuring temperature or humidity, etc.
The project is now based on Espressif's SDK version 1.0.1b. More details about the project can be found in the Wiki.