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 nice bonus, it even includes a programmable gain amplifier, up to x16, to help boost up smaller single/differential signals to the full range. We like this ADC because it can run from 2V to 5V power/logic, can measure a large range of signals and it's super easy to use. It is a great general-purpose 12-bit converter.
The address can be changed to one of four options (see the datasheet table 5) so you can have up to 4 ADS1015's connected on a single 2-wire I2C bus for 16 single-ended inputs.