This morning, while riding my bike, I thought, "Wouldn't it be nice to know when someone is approaching from behind?" I didn't want to use mirrors or funny glasses, so I decided to use an ultrasonic transducer to trigger a buzzer when approached.
This involves a Ping transducer, Arduino, buzzer, switch, 9 volt battery and enclosure.
Thanks to the world of 3D printing, these things don't have to be force fit into a black box anymore.
For the design and print files of the box (created in Autodesk's 123d software--free): http://www.thingiverse.com/thing:115074