Are there free operating systems?

Yes. A very large variety of operating systems are available free of cost, and many with source code available.

Operating systems available free of charge, for any purpose, with source available:


  • Linux - Unix-like OS
  • FreeBSD - Unix-like OS
  • OpenBSD - Unix-like OS
  • NetBSD - Unix-like OS
  • FreeDOS - MS-DOS clone
  • ReactOS - Windows 2000 / XP clone (in alpha, not suitable for daily use)
  • Haiku - BeOS clone (in alpha, not suitable for daily use)
  • AROS - AmigaOS clone (lacks a web browser and good device support)
  • KolibriOS - Floppy-size OS with a crude GUI
  • Syllable - general desktop OS; Amiga-like.
  • OpenSolaris - open-source codebase of Solaris (see below)


 

Operating systems available free of charge for personal use, without complete source:

  • Solaris 10 - Unix implementation from Sun Microsystems. Useful as workstation / server; lacks comprehensive hardware support.
  • BeOS Personal Edition - multimedia-oriented desktop; doesn't work on most newer computers.
  • DR-DOS - MS-DOS clone
  • MenuetOS - Floppy-sized OS with GUI.
  • QNX - realtime / embedded OS; highly responsive environment, though mostly lacking in applications (you were supposed to write your own!)
 

Answer:

Why you buy costly OS from Microsoft or some other companies. There are some othe popular operating system that work same like Microsoft' OS. Most of the OS are freely available to download and run on private machines.
Some are
1. DOS
2. RxDOS
3. Ubuntu
4. ReactOS

etc

You can find and download more free OS from the link that I have posted in the related links with this question.