I'm sure you are aware of open source software, but there is also open source hardware. This is where the maker of some electronic hardware publishes the design and schematics publicly so that anyone can make (and sell) the hardware. You will find many breakout modules (such as the ADS1115 modules we used in Chapter 5, Connecting Your Raspberry Pi to the Physical World) from various vendors with different (or no) branding. Different vendors may also make their modules in different colors and, while less common, different physical layouts.
The core or heart of a module – particularly the more simple ones – is often a single integrated circuit (IC or chip). As long as the core IC and I/O pins are similar, it's generally safe to assume that boards will operate the same way.
SparkFun (https://www.sparkfun.com/) and Adafruit (http://adafruit.com/) are two companies producing open source hardware that many others clone...