Linux
Linux is a Unix-like kernel used with the GNU operating system, commonly referred to as the GNU/Linux system. 1
There are many different variants of the GNU/Linux system, often called distros. 1
Linux is incredibly flexible, as it can run on any hardware. You can probably find Linux running in most electronic devices.
Due to Linux's open-source nature, it is highly customizable and users have complete control over every aspect of the system.
Linux is popular amongst software developers for being light-weight, highly customizable, and highly compatible with most software and hardware. The features and tools that come with Linux give developers more power and control than other operating systems.