I keep an empty water pitcher on the counter and whenever I am cleaning up during the day I dump abandoned glasses of water, all but finished plastic water bottles (you know the ones that get left in the car so you don't want to drink them...) into the pitcher. When it gets full I use it to water the plants outside. (I was surprised how quickly that happened and then horrified that I was wasting that much water in such a short amount of time.)
This seems like such a simple thing, but I think many people overlook this simple yet effective way to use the water that would normally be wasted down the drain.
I hope this helps everyone to think twice before tossing that water.
p.s. If you don't have a plant to water, I suggest checking out all the great instructables on gardening and get one :)