AquaOne Technologies’ H2Orb will let you know when you’ve got a leak or an overflow in your toilet. It attaches directly to the water valve behind the toilet, with a single float sensor that hangs inside the toilet tank. When it detects a leak, it shuts off the water and alerts you of the leak with an alarm as well as an icon on its LCD screen. The entire system runs on a single 3V lithium coin cell battery.
This device is great for those who live in places with limited resources, not to mention saving water. The sooner you know about a leak, the better.