Rain harvesting
Rain harvesting is a great addition to just about any house. In a residential area, it can be used for toilet flushing, for the garden, and for drinking (with resource consent). In baches and rural areas, it may provide the main source for all water. A good overview is available through the Smarter Homes website.
UV treatment
If you intend to drink collected rainwater, some sort of treatment is needed. This could be boiling, chlorination, or filters. The most modern solution however is a UV treatment system, which uses ultraviolet light to kill mirco-organisms. Sue Kelly has this type of system available for sale.