If you want to more easily see the road behind you from your motorcycle, hacker Nescioqd shows us how he mounted a display inside his helmet and connected it to a rearview camera on his bike.
Nescioqd's project was pretty complicated, but some of it seems unnecessary if you'd rather not work with electronics so deeply. His particular hack not only mounted a camera on the back of his bike, but he routed the power supply for the tail light to the camera as well. It's clever, but you can probably just run it on batteries if you'd rather not mess with it.
The display is a small LCD screen shaped like glasses lenses that go near your eyes (so you can tell when you're going over 9000). It's a clever idea, though not without its problems—you'd need to make sure you got the right kind of helmet and screen so you could see it at night. Hit the link for more details.