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:

1. Running classic DOS games
2. Running business software that only supports DOS
3. 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.

Click Here to Download ReactOS

OpenSolaris

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).

FreeBSD

FreeBSD is an advanced operating system for x86 compatible (including Pentium® and Athlon™), amd64 compatible (including Opteron™, Athlon™64, and EM64T), ARM, IA-64, PowerPC, PC-98 and UltraSPARC® architectures.  It is derived from BSD, the version of UNIX® developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.
FreeBSD offers advanced networking, performance, security and compatibility features today which are still missing in other operating systems, even some of the best commercial ones.
FreeBSD makes an ideal Internet or Intranet server. It provides robust network services under the heaviest loads and uses memory efficiently to maintain good response times for thousands of simultaneous user processes.

Debian Linux

Debian is a free operating system (OS) for your computer. An operating system is the set of basic programs and utilities that make your computer run. Debian uses the Linux kernel (the core of an operating system), but most of the basic OS tools come from the GNU project; hence the name GNU/Linux. Debian GNU/Linux provides more than a pure OS: it comes with over 18733 packages, precompiled software bundled up in a nice format for easy installation on your machine.

Fedora Linux

Fedora is a Linux-based operating system that showcases the latest in free and open source software. Fedora is always free for anyone to use, modify, and distribute. It is built by people across the globe who work together as a community: the Fedora Project. The Fedora Project is open and anyone is welcome to join.

Koppix

KNOPPIX is a bootable CD or DVD with a collection of GNU/Linux software, automatic hardware detection, and support for many graphics cards, sound cards, SCSI and USB devices and other peripherals. KNOPPIX can be used as a productive Linux desktop, educational CD, rescue system, or adapted and used as a platform for commercial software product demos. It is not necessary to install anything on a hard disk. Due to on-the-fly decompression, the CD can have up to 2 GB of executable software installed on it. (over 8 GB on the DVD “Maxi” edition).

Mandriva

Mandriva Linux is a community-based Linux distribution, suitable for a wide range of situations from typical desktop use through development and server roles up to clustering.
 
Mandriva Linux is based on the Cooker development project sponsored by Mandriva. Hundreds of passionate free software developers work openly on the core of the distribution. This open, community-driven development system has been in place since 1998, making it one of the longest-standing open source development communities around. The involvement of the Mandriva development community helps us to make Mandriva Linux one of the largest, most up-to-date, integrated, internationalized and standardized distributions available.

Slackware

Slackware Linux is a complete 32-bit multitasking “UNIX-like” system. It’s currently based around the 2.6 Linux kernel series and the GNU C Library version 2.7 (libc6). It contains an easy to use installation program, extensive online documentation, and a menu-driven package system.

A full installation gives you the X Window System, C/C++ development environments, Perl, networking utilities, a mail server, a news server, a web server, an ftp server, the GNU Image Manipulation Program, Mozilla Firefox, plus many more programs. Slackware Linux can run on 486 systems all the way up to the latest x86 machines (but uses -mcpu=i686 optimization for best performance on i686-class machines like the P3, P4, Duron/Athlon, and the latest multi-core x86 CPUs).

Ubuntu

Ubuntu is a community developed operating system that is perfect for laptops, desktops and servers. Whether you use it at home, at school or at work Ubuntu contains all the applications you’ll ever need, from word processing and email applications, to web server software and programming tools.
Ubuntu is and always will be free of charge. You do not pay any licensing fees. You can download, use and share Ubuntu with your friends, family, school or business for absolutely nothing.

PS: This is by no means a complete list of Free operating systems… but that is all for today folks… I will be updating this list through out this week. My aim is to make this the best resource for finding a free alternative to Microsoft Windows and other proprietary Operating systems.