UNIX/Free BSD

Developed in the late 1960s, UNIX was created by some of the members of the Multics team at the Bell labs, many of whom helped create the C programming language.

The UNIX of current time is not just the work of a few programmers. Other organizations, institutes and various individuals made significant additions to the UNIX system.

Primarily a command line oriented OS, additional applications (such as X-Windows) allows you to use a GUI system (similar to Windows 3.x / Windows 95 / Windows 98) . While this is available, UNIX is still primarily used from the command line.