A web-enabled extension to your home lawn irrigation controller that intelligently enables or disables your sprinkler system.
The SprinklerSwitch project has FINALLY been updated to support the latest version of Raspian. The biggest hiccup was switching to the WiringPi2 library - which is now the defacto library for addressing GPIO pins. The good thing is pin addressing is now entirely in userspace (no sudo required!), however there were significant changes that required refactoring.
Things are up & tested with the latest version of Raspian however - so have at it!
First, construct a switch for your Raspberry Pi GPIO pins. I generally use MOSFETs for these low-voltage operations, with the gate to a GPIO pin.
Some sprinkler systems have a moisture / rain sensor that open or close a circuit to disable or enable the next scheduled irrigation. Usually if a sensor is not installed, there's a jumper wire to keep the circuit open. Replace the jumper or sensor wires with the open & ground wires on the relay or MOSTFET wired to the Pi. --> DO NOT WIRE THE GPIO PIN TO YOUR SPRINKLER SYSTEM <--. You'll likely frizzle-fry the whole thing. You want to close the circuit, not send any amperage to it. Seriously. Don't move on unless you understand your irrigation system. SERIOUSLY.
If necessary, enable the sensor or disable the sensor bypass on the irrigation controller.