iStrategy Labs' Dorothy is a mobile app and Bluetooth-based switch (called the Ruby) that slips into your shoe. Click your heels together three times and it triggers an action on your smartphone like calling an Uber.
Dorothy is powered by the LightBlue Bean, an Arduino micro-controller with a built in Bluetooth chip, accelerometer, and coin cell battery. Simply clip the "Ruby" into your shoe, connect it to the Dorothy iPhone App, and select your desired action after a successful trigger (3 heel clicks). The onboard accelerometer sends a signal to your phone via Bluetooth (listening for spikes in the data over a short period of time).