If you are interested in using Arduino to control water valve (connected to garden hose etc.), this may be useful -- minty water valve controller (link given in the next message). It controls an off-the-shelf Orbit yard watering valve (available in many home improvement stores, at $14.95 each), and the circuits fit entirely in a mint tin. Powered by li-poly rechargeable battery, and also features wireless control.
Link for the above post:
To save you looking, and to highlight something I for one missed...
The above takes you to a complete Arduino based project... lawn (or other) watering, on/off controlled by Arduino. Wireless control from afar... to change/ over-ride settings, I believe. I believe it will just follow a schedule in the absense of new wireless commands.
The water control valve at the heart of it costs about $16, seems to be available in US and Europe. It has garden hose fittings. Easily used in your own projects, independent of the fancy stuff already invented by Ray.
And, quoting from the complete project:
+24v opens the valve, and -24v closes the valve. It uses a latch solenoid, drawing power only when you open or close it.