WebJack is a JavaScript library that uses an audio modem to communicate with an Arduino µC via headphone jack. No need to install anything.
Make sure the audio cable is connected and your Arduino is running one of the example sketches. Eventually, crosstalk between signal lines will make sent data appear as received data. To test without an Arduino, use this YouTube video recording of sensor data
WebJack is in an early development stage. If it does not work, try to:
use an other browser (Safari will not work, sorry)