The precision voltage shield is an 8-channel analog-to-digital converter that plugs into Arduinos, Rascals, and anything else with Arduino headers. (Running it with the Arduino Mega requires adding a few jumper wires, but it works.)
The software to run the shield on an Arduino, written by Brandon Stafford of Rascal Micro, is available for free under a BSD open source license. You are welcome to use the code for whatever purpose you like-- use it in your own projects, sell it in a product, whatever you like. At present, the shield can measure 8 channels at 735 Hz when plugged into an Arduino Uno loaded with the Arduino software.
The shield is based around the Analog Devices AD7606 chip. The resolution of each channel is 0.15 mV, which is better than most commercial multimeters.
More details about the development of the shield software and the shield's specifications can be found on the Precision Voltage Shield's documentation page.