I've had a recurring idea to build a water recycling shower. The idea being the water is initially stored in a tank below the shower, pumped up through the shower head and returned to the tank via the drain. A (hopefully) small amount of hot water is added to the tank to maintain the temperature of the water and to introduce more clean water. Some degree of filtering would obviously be required on the return path, at the very least to remove hair which would damage the pump but hopefully also removing soap suds etc.

I'm not sure if it would really work but it seems like it would be ideal for some situations, for example in motorhomes, small yachts etc.