You hear occasional stories of people where they get a big settlement by a waterleak. People today are more than ever concerned with their water consumption. 
The media gives message that the municipalities will increase the charge per litre. 

I now have a very cheap solution which monitors your watermeter for normal water usage. 

The user gets an alarm if in 24h there is no 2h where no (less than 0, 5 l) water is used. If this is not the case, there is a water leak. 
If the user leaves open a watertab longer than 30 min then there is also an alarm. In this case one has forgotten the water tap. 

The circuit consists of an Atmel attiny 13 powered by 2 AAA bateries. The design would consume about 5µA and thus would last for many years before new bateries are required.