Condensation...it's everyone's first science experiment. Place a cold thing in a warm environment, and water vapor gets cooled to its dew point.
So, what's a big cold thing in an even bigger warm environment? Well, your air conditioner, for one, and it condensates with the best of them. Translation: free water, perfect for giving your garden a drink.
From DIYNetwork: "No matter what kind of air conditioner you have, whole house or window box, it drains water that you can capture. Each time an air conditioner runs, it pulls moisture from the air inside the home. Since the water has to go somewhere, it's drained outside the house, usually through a condensation pipe. The water slowly drips from the pipe each day. It doesn't seem like a lot of water, but the constant dripping really adds over time. This water can be caught in a bucket or fed directly to nearby plants." Done!