Here in Belgium it is mandatory (for quite some years) with every new building development to install a tank capturing rain water from the roof. This water must be used for flushing toilets and you typically also use it to water the garden, wash your car, etc. With some extra (carbon) filters people also use it for their washing machine.

I have a 10000L tank which is more than sufficient for a household of 4 and some garden. The minimum size is calculated based on your roof surface but normally people install a bigger tank. The extra cost for a few 1000L more really isn't that much and it can save you the cost of a fallback system in case the tank runs dry.

The overflow of the tank goes to a separate sewage system that usually ends in a local stream. This way it doesn't dilute the actual grey/black water which, incidentally, makes a water treatment plant more effective (or so they say). It also allows the water to infiltrate into the ground and there's less risk of flooding the streets during a downpour. At least, this is the theory. Work is still very much in progress to split the sewage systems but we're getting there.