iButton or 1-Wire is a very simple network which runs over almost any single wire medium and shared ground wires. It is used for simple device identification, temperature probes, time keeping, outputs all the way to advanced user authentication and even communications between independent micro computers.
My primary use for it is in my house to check temperature, detect button presses or motion detectors and control indicator lights.
The greatness of this product lies in the price. The controlling serial interface is about US$10 (about AUS$37, cheaper if importing). A simple identification chip (guaranteed unique in the world with a 48 bit id) is about US$1, and more complicated chips like analogue inputs and temperature chips are around US$5.
Hardware::iButton was written by Brian Warner and can be found at http://www.lothar.com/tech/perl/.
I am working on a new version which adds some new hardware and updates the serial drivers.
Original release - http://www.lothar.com/tech/perl/
CPAN release (current) - ?