You don't really need to care how fast or slow you're going on a bicycle, but careening through traffic at a blistering 13km/h gives some people a thrill. If that describes you, check out Matt Richardson's Dynamic Bike Headlight hack.
Using a Raspberry Pi at its core, Matt turned a compact pico projector and a handful of other electronics into a bicycle headlight that does so much more. Well, not that much more. In addition to providing a little extra light on a dark road, the projector also displays the bike's current speed.
Now, if only cars had this feature, since it would probably shame drivers into sticking closer to the speed limit if they knew everyone could see how fast they were driving. [YouTube via Reddit via Damn Geeky]