The Bicycle Speed-Vest is great way to let drivers around you know where you're at while showcasing your current speed in a bright, illuminated manner. It's actual pretty practical, though it might draw too much attention to yourself.
According to talk2myshirt:
The complete system consists of a wheel speed sensor mounted on the bike, the numerical display and the open source computing platform Arduino which is doing the speed calculation and display control. the power comes from a 9 volt battery for the Arduino and one AA battery for the display, both running for 6 hours on one set of battery.
Seems like a pretty cool way to keep others on the road aware of you. Safety first, as they say. And while it could be a good way to see how you're doing on your cycling speed, you might have some trouble seeing the bright display for yourself.