LEAP#181 Voltmeter Modules
There are two and three wire modules in the market. Two-wire modules are the simplest (and generally cheapest). For most applications where a simple readout of a power supply is required, they are most convenient since a separate power supply connection is not required. It does mean of course that the meter draws current from the circuit under test, and will only work down to a certain voltage (usually ~4.5V).
For more precision measurement, the three-wire modules are best as they present relatively high input impedance (> 100kΩ), and can measure voltages all the way down to 0. As an example, Adafruit have this 3-wire module in their catalogue, but they are widely available from most online sellers.
I recently received a 3-wire module as part of a kit and put it to the test. Using my DMM as a reference, it appears accuracy is within 40mV, good enough for general purposes.
As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub.