The Microchip® MCP3008 Analogue to Digital Converter [Datasheet] is commonly used to add analogue (or analog for US readers) inputs to the Raspberry Pi. It is very easy to connect to the Pi using an SPI bus on 4 of the Pi's GPIO ports. There is plenty of software support for it, but it is also easy to bit-bang from a program if you want to drive it entirely yourself.
This is a presentation I gave at Raspberry Pint in London on 24th April 2018 on using the MCP3008 on the Pi, including a live experiment to see how fast I could make it run (how many samples per second). My next blog entry looks at an application design which includes an MCP3008 used in a novel way.
I would like to thank CodeNode for making the video available.
I do not represent or speak for Microchip®.