by Peter Boxler :
Native Raspberry Pi does not have an On/Off switch and there is no easy way to shutdown the Pi while keeping the filesystem intact. This Intelligent Power Switch allows just that: Power-On the Pi by pressing a pushbutton and also properly Power-Off the Pi with another press on the same button. The intelligence is provided by a program running in an AVR MCU ATtiny44. This C-program implements a Finite State Machine in the MCU. A small Python script is running in the Pi itself. Just one GPIO-Pin is used for two-way communication. In addition, a variable frequency square wave is available for externally interrupting the Pi.
iSwitchPi Adds an Intelligent Power Switch to Your Raspberry Pi – [Link]