Rice is cooked in water. When done, you're left with a bowl full of foul-tasting, white water. Instead of throwing it away, water your plants with it.
Rice contains starch, which gives the rice water its opaque, white color. These starches are beneficial to plants, encouraging healthy bacteria that grow in the roots.
Rice includes nitrogen, potassium and phosphorus, trace amounts of which are found in rice water. These minerals are also beneficial to plants.
What Not to Do
Don't water your plants with hot water -- wait for it to cool first. If you've salted your rice while cooking.