Rainwater is beyond question, the best water source for watering plants. It is low in salts and contaminants. Consider rain barrel to make the most of light rains and to store extra water for dry periods.
The water in your freshwater fish aquarium is rich in nutrients for plants. Save the water when you clean your freshwater aquarium. Do not use the water form salt-water aquariums.
Any outdor fresh water source, such as a lake, pond, river, creek or fishpond,is a great source of water for watering plants. Avoid water downstream of industrial pollution sources; if there are lots of living plants and/or animals in the water, it's probably safe.
If you run an electric dehumidifier, the condensate is reasonably pure water. Periodically scrub out the collection container to prevent microbes being established.
The water you use to blanch or cook food is often rich in vitamins and minerals that can benefit plants, as long as the cooking water does not contain more than a trace of salt, oil, sugar, or starch.