I am interested in obtaining an ADC (Analog to Digital Converter) or Analog input for the Raspberry Pi. I understand that various boards exits to use an Arduino as input to the Pi and that Libellium has an arduino-like adapter containing Analog inputs as well as in theory the I2C and SPI interfaces possible with ADC chips. Does anyone have a daughter board with multiple ADC inputs, and/or is their a software library for any of the available USB data capture devices that would meet this neet without considerable interfacing expertise and level shifting?
A good starting point is adafruit's tutorial at http://learn.adafruit.com/reading-a-analog-in-and-controlling-audio-volume-with-the-raspberry-pi/overview. Adafruit also has breakout boards for the RasPi to make things a little easier... This post is also useful: How can I connect an analog to digital converter (ADC)?
The MCP3008 seems to be one of the more popular ADC's for the RasPi and it uses the SPI interface for communications
If you want to attach an Arduino via USB and read out its ADC's check this posting: How to attach an Arduino?