DroidScriptBusPirate is a User Inferface for the BusPirate^ (pocket electronics bench tool) which runs in the DroidScript app on Android phones. It simplifies talking to the BP (BusPirate) via USB OTG by providing a list of commands and setup wizards for the commands which require parameters. As a result, you don't have to remember the cryptic command codes.
BusPirate is a little open source device that connects a USB host device (PC, MAC, or cellphone / tablet via USB OTG) to electronic stuff. It can speak with several different busses (I2C, SPI, UART, etc...) and devices (LCDs, servos, AVRs, FPGAs, CPLDs, etc...) and can inject signals, or measure voltages. It's a little test bench in your pocket for less than $30.
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.