Have a complete, although very basic, electronics bench in your pocket on the cheap.
Big update! Adds a scaling factor to the value displayed in Voltmeter mode, and a calibrate button which powers up the internal supplies and explains how to connect a potentiometer so that the input voltage can be scaled, and the +5 power supply can be used as a voltage reference to calibrate the potentiometer.
In order to automatically turn on the power supply, the app now tracks the BusPirate mode, and if it's in HiZ, it automatically switches to 1-Wire (can't turn on power in HiZ) and then switches back when finished.
So then you can connect the external voltage through the pot as a resistor divider and read the actual voltage on the display, when the voltage is more than the 6v range of the BusPirate ADC. E.g. you can measure a 12 or 24 or 100 volt signal without frying the thing.