A keychain-sized flashlight that uses a hi-intensity LED instead of a bulb. It comes in many classic LED colours, as well as a very cool bluish-white colour, very similar to HID headlights seen on expensive cars.
The design is very simple: Two coin-sized lithium batteries stacked between the leads of an LED housed in a plastic case with a little switch that breaks the contact between batteries and LED (why didn't I think of that).
Because LEDs are very efficient at converting electricity to light, they are said to be able to light continually for about a week.