An open-source electronic device could be potentially useful for cyclists.
Velosynth is an open-source, hackable sound synthesizer that attaches to a bicycle and interprets its speed and acceleration into a useful audio feedback for pedestrians, fellow cyclists, and other people sharing the road. The device contains a magnet and a sensor attached to the wheel to measure speed and acceleration, in addition to having a built-in amp, an accelerometer and an Arduino-compatible microcontroller. Velosynth attaches to the bicycle with two bungee loops that wrap around the top and down tubes of the bicycle.
A DIY Velosynth kit is available for $100.
Watch a video demo below: