OpenEarable is a new, open-source, Arduino-based platform for ear-based sensing applications.It supports a series of sensors and actuators: a 9-axis inertial measurement unit, an ear canal pressure and temperature sensor, an inward facing ultrasound microphone as well as a speaker, a push button, and a controllable LED. We demonstrate the versatility of the prototyping platform through a series of example application scenarios. As such, OpenEarable offers a general-purpose, open sensing platform for earable research and development. Full information under https://teco.kit.edu/open-earable/