There are an increasing number of equipment manufacturers and technology groups that provide operating systems (OS) for free personal and commercial use without any cost or fee. You are sometimes even free to use them, change their source code and redistribute them as well.
Like Anything that comes for free, do not expect fancy printed manuals and CD-ROMs and technical support. Yet you can count on the community that uses these OS to provide you support through chat rooms and forums and mailing lists.
You will also note that the size of these OS range from 1 to 10 GB so if you don’t have decent broadband connection, paying for a DVD/CD-ROM might seem reasonable. Also if you are not that techie, some of these companies offer helpdesk and support contracts, which can be bought at a reasonable price.
FreeDOS
FreeDOS aims to be a complete, free, 100% MS-DOS compatible operating system (mostly achieved except Windows compatibility – Windows standard-mode works on FreeDOS, but 386-mode / WfW 3.11 does not.)
These days, there are three main uses for FreeDOS:
- Running classic DOS games
- Running business software that only supports DOS
- Supporting embedded DOS systems, such as a computerized cash register or till
FreeDOS should run on any standard PC, but if you are new to DOS, we recommend you use a PC emulator to install and boot FreeDOS. You can find PC emulators for all computer platforms (Windows, Linux, Mac.) You can even run FreeDOS using a PC emulator written in Java, right in your web browser!
RxDOS
RxDOS is a fast MS-DOS clone that supports very large disk drives, FAT32 volumes, and Windows 95/98 Long Filenames. its source code is also available free at sourceforge.
React OS
ReactOS is a free, modern operating system based on the design of Windows® XP/2003. Written completely from scratch, it aims to follow the Windows® architecture designed by Microsoft from the hardware level right through to the application level. This is not a Linux based system, and shares none of the unix architecture.
ReactOS had its origins in an older project known as FreeWin95. FreeWin95 aimed at being binary compatible with Microsoft® Windows® 95. It was founded by Yannick Majoros around 1996. But the development never took off substantially due to endless talks on design and thoughts of following the NT architecture.
The main goal of the ReactOS project is to provide an operating system which is binary compatible with Windows. 
This will allow your Windows applications and drivers to run as they would on your Windows system. Additionally, the look and feel of the Windows operating system is used, such that people accustomed to the familiar user interface of Windows® would find using ReactOS straightforward. The ultimate goal of ReactOS is to allow you to remove Windows® and install ReactOS without the end user noticing the change.Please bear in mind that ReactOS 0.3.7 is still in alpha stage, meaning it is not feature-complete and is not recommended for everyday use.
OpenSolaris
OpenSolaris is an operating system (OS), an open source project licensed under CDDL, and a community. The project’s goals are innovation, collaboration, and the extension of OpenSolaris technology.OpenSolaris is free, open source, and well-suited for desktops, laptops, servers, and data centers. The quality requirement of OpenSolaris is perhaps best stated as Production Ready All The Time.
OpenSolaris technical communities maintain kernel and userland consolidations and launch new technology projects. OpenSolaris is developed by communities working in the different projects. Each project focuses on one or more specific areas of the system. The OpenSolaris developer project is sponsored by Sun Microsystems, Inc.
Darwin
Darwin is the UNIX technology-based foundation of Mac OS X. Darwin integrates several technologies. Among the most important are
4.4BSD-based operating-system services (built on the Mach 3.0 microkernel), the I/O Kit, networking facilities, and support for multiple integrated file systems. Developers can use Darwin to port UNIX/Linux applications and create kernel extensions.
In addition to being part of Mac OS X, Darwin is a standalone, BSD-based operating system. (BSD, short for Berkeley Software Distribution, is a family of UNIX variants descended from Berkeley’s version of UNIX.)Darwin is also occasionally used to refer to the Darwin Streaming Server, also known as the QuickTime Streaming Server (QTSS).