For microcontrollers without an analog-to-digital converter or when you want a higher-precision ADC, the ADS1015 provides 12-bit precision at 3300 samples/second over I2C. The chip can be configured as 4 single-ended input channels, or two differential channels. As a bonus, it even includes a programmable gain amplifier, up to x16, to help boost up smaller single/differential signals to the full range. This ADC is great because it can run from 2V to 5V power/logic, can measure a large range of signals and it is easy to use. It is a great general purpose 12 bit converter.
Interfacing is done via I2C. The address can be changed to one of four options, so you can have up to 4 ADS1015s connected on a single 2-wire I2C bus for 16 single ended inputs.
For starters, there is an example code by Adafruit for both Raspberry Pi (in Adafruit Pi Python library) and Arduino (in Adafruit ADS1X15 Arduino library repository). Simply connect GND to ground, VDD to your logic power supply, and SCL/SDA to your microcontroller's I2C port and run the example code to start reading data.
Raspberry Pi Example Code: https://github.com/adafruit/Adafruit-Raspberry-Pi-Python-Code
Arduino Example Code: https://github.com/adafruit/Adafruit_ADS1X15
Adafruit Product Page: https://www.adafruit.com/products/1083