Linux is a prominent example of free software and open source development where all underlying source code can typically be freely modified, used and redistributed by practically anyone. The Linux kernel was first released to the public on September of 1991 specifically for the Intel x86 PC architecture. Its operating system however, is UNIX-like, which was a system that was originally conceived and implemented during the 60s.
Linus Torvalds decided to work on a non-commercial replacement for MINIX whose modification and redistribution were restricted. The product of such effort was the Linux kernel. It was augmented with system utilities and libraries from the GNU project. Developers of Linux and GNU worked for integration of GNU components with Linux to come up with a fully functional and free operating system.