I love the idea of using rainwater to do other things. A lot of the time it lands on our sidewalks and streets only to run into the gutter, then sewer, then with the rest of the garbage water to the treatment plants. But, most of the time the water that falls from the heavens is perfectly good water. Here are some ideas for ways to use it.
1) Holding for watering plants - Now, this is what the rain was meant to do, right? Provide water to sustain life on this planet. You can set up a fancy collection system, or you can simply put out a big bucket (make sure some mesh is over it so that bugs and animals don't get in) then let the rain fill it up. If it is small enough you can water with it, if not, use a watering pitcher to water with.
2) Go the bathroom - In some places you are allowed to rig up a system to flush your toilet with rainwater. We all know how wasteful toilets are (in regards to water...) but using rainwater reduces that water used. You are just using it to flush the toilet before it makes it to the sewer.
3) Put it in your radiator in your car - My car overheats all the time. I have tried to have it fixed, but alas, it still does it. (Suggestions are welcome on how to fix it.) Because of this, I have a large collection of water in my trunk in case I need to replace the water or cool it down. The nice thing about it is that I know I could drink it as well. I even have water for my friends when they are with me. However, it is a waste when I do need to cool my car, very depressing actually. So, moral of the story, fill up your radiator first. Put a small bucket out in the rain (or even on your driveway where the sprinklers hit) then pour it into your radiator that day. It should keep your car stocked as well as use the water for something productive.
4) Wash your windows - Now, I have news for you, most of the time when washing windows, it is not the product you use as much as how you do it. The fancy cleaners, with all the bad things in it, don't really do that much better of a job than water. I have heard that newspaper instead of a cloth works great, have yet to try it. I am also a fan of squeegees. When I wash my car however, I just use a towel before the water drys to dry off the windows. Works great.
5) Water fight! - My mother is the best at starting water fights then getting away from them unscratched. We've had water fights at family gatherings, at neighborhood parties, and even just in our own yard. It use to be a yearly tradition. Now instead of using all your good water to throw at people, use rain water instead. If you use water balloons, you can premake a bunch of water filled goodies and stock pile them until the time is right for attack.
Any more you can think of?