Rain water can definitely be a water source in an emergency (assuming that it rains). In fact, many homes around the world already have cisterns, sometimes on their roofs, to provide water where water supplies are already limited. Rain water, however, is usually pretty dirty and needs to be filtered using one of the methods that we've already discussed. A large barrel at the bottom of a rain gutter would be a great source of water for gardening. You can buy barrels especially for this purpose. Typically they also have a tap at the bottom. A full barrel can provide enough water pressure to hook up a hose.
A video on how to make a rain-water storage barrel: