This four channel Arduino software voltmeter with history was made by using an Arduino Uno and an application written in the Processing language that runs on a computer.
The Arduino reads analog values on four analog input pins connected through voltage dividers and then calculates the voltage. The Processing application displays the voltage reading and the voltage history is displayed on a graph for each channel.
This image shows the software voltmeter application:
Click the above link for the circuit diagram, Processing source code and Arduino source code for this project.