Disposable Electric Toothbrush - scrub out the sinks, especially detailing around the drains. You'll never have this thing touch your teeth again, but it sure beats scrubbing mildew and grout with the manual type of toothbrush.
Sponges - cut them up into small strips, microwave them (to kill any bacteria) and use them to clean inside of wine glasses, where a normal sponge is just too big.
Cardboard Boxes - flatten them, and create a mat underneath your car to capture any liquids that might drip, including rainwater (which is usually loaded with oil and road grime), and will stain the floor of your garage or driveway.
Cardboard Tube Inside Paper Towel Roll - make your dog's day, give it to him as a toy but make sure to take it away before he decides to eat the mushy goodness. Skip this last step at your own peril.
Have more ideas about how to get just a little more functionality out of everything? Tell me!