Taking really long showers is a great way to waste a whole lot of water. But it's so easy to lose track of time in there, what with it being so warm and nice. You just need a reminder to turn the shower off.
The Waterpebble does just that. It sits on the floor of your shower and flashes an LED light at you so you know how much water you've used. It goes from green to yellow to red, telling you when it's about time to get out and just when you go from simply cleaning yourself to becoming a water-hogging eco-destroyer. It's a subtle yet important distinction.