When it comes to desktop operating systems, there are three obvious choices: Windows, Mac OS X, and Linux. But a whole world of alternative OSes lies below the mainstream radar.
These little-known products are actively or recently developed, and some folks actually use them to get things done. Here are twelve of these strange beasts, all of which run on modern x86-based PC hardware, and many of which can be downloaded for free. Impressively, none of them are based on Linux.
GEM (Graphical Environment Manager) began its life in 1985 as Digital Research's graphical shell for CP/M. It made its way to MS-DOS and was the Atari ST's interface. Caldera, which ended up owning GEM, open-sourced it in 1999. Since then, enthusiasts have updated and extended the code, which is typically released as FreeGEM or OpenGEM.
You can download an OpenGEM package that runs on just about any PC and includes apps-I recommend the "Full CD" package of FreeDOS, a DOS workalike that comes with OpenGEM. Just don't expect too much: GEM retains its primitive look-and-feel so it can run on old hardware.
Syllable is a free, open source OS with a strange pedigree. It began in 2002 as a fork of AtheOS, an earlier open source operating system (no longer developed) that once aspired to be an AmigaOS clone. Syllable uses the AtheOS file system (itself similar to the BeOS file system) and retains partial POSIX compliance that makes it easier for Syllable enthusiasts to port Linux applications over to the platform if desired.
The ultimate result is an operating system that is neither here nor there and has very little application support. But its developers keep chugging along with the goal to provide a free, easy-to-use desktop OS that is not based on Linux.
As a capable graphical OS, MenuetOS pulls off a remarkable feat: it can be booted and run off a single 3.5″ floppy disk. Of course, if you want a more sophisticated setup, you can install it on a hard drive.
A key feature of MenuetOS that lets it run in such limited space is that it is written entirely in x86 assembly language. The author's dedication to assembly-only programming keeps the OS and its included apps lean and speedy, and these properties have attracted a small but dedicated following over the past decade.
Breadbox Ensemble traces its ancestry to PC/GEOS (cousin of the more well-known 8-bit GEOS), released by Berkeley Softworks in 1990. Like GEM, the environment failed to catch on in the face of Windows' success, so ownership of the OS changed hands over the years while maintaining a user base.
Ensemble, in its current configuration, looks and feels much like a cousin of Windows 95, and its capabilities are roughly equivalent. Unlike most of the operating systems on this list, it's not free software. Breadbox Computer Company still sells it from their website for $99.95 a copy.
Inferno began in 1995 as a research operating system at Bell Labs. Its designers wanted to incorporate lessons learned in an earlier Bell research OS playfully called "Plan 9 From Bell Labs." Both are notable for being distributed OSes, which seamlessly allow users to utilize computing resources distributed across a network of machines.
Inferno differs significantly from Plan 9, however, by its inclusion of a virtual machine that ensures applications and the user interface are consistent across all platforms (which include x86, ARM, PowerPC, and SPARC). Inferno can be downloaded for free from its official website.
After 1996′s OS/2 Warp 4, IBM's OS/2-the OS that was supposed to replace DOS-fell off the radar for most PC users. Behind the scenes, however, loyal users began to clamor for updates and upgrades. Within a few years, a company called Serenity Systems-with permission from IBM-began to deliver them. Serenity wrapped its OS/2 upgrades and OS/2 itself into a new OS distribution called eComStation, which Serenity still actively develops today. eComStation, currently at version 2.0, sells for $149 (Home Edition) to $219 (Business Edition).
Seal GUI for DOS
While Microsoft abandoned plain old MS-DOS back in 1995, that hasn't stopped die-hard users of the classic command line OS from keeping its spirit alive. A number of Windows-like graphical shells are available for MS-DOS and its workalike cousins (notably FreeDOS and DR-DOS) - we've already seen one in the form of OpenGEM.
Another well-regarded and capable GUI for DOS is Seal, a colorful 32-bit windowing system with low system requirements (it runs on a 486 CPU and up). It's also free, open source, and tweakable.
KolibriOS is a small and speedy assembly-based hobby operating system that forked off of MenuetOS code in 2004. Like MenuetOS, this free, open-source OS can be run off a single floppy disk, but is also capable of expanding to meet larger needs on a hard disk installation. While Kolibri and Menuet are similar under the hood, each subscribes to a different user interface philosophy. Each is equally capable in its own way.
It's rare that someone undertakes a project to write a completely new x86-based operating system from scratch and sell it as closed source, proprietary software. But that's exactly what happened with SkyOS, which began as a simple bootloader program by Robert Szeleney in 1996. Over the next 8 years, Szeleney transformed it into a full-blown graphical OS with its own file system and API. He aimed to sell it and launched a paid beta program in 2004. Around 2008, updates began to dry up due to changes in Szeleney's life. Currently, development for SkyOS is on hold as Szeleney ponders the fate of what he has created.
If you've ever wanted to run Windows without actually running Windows, you're in luck. ReactOS is a free, open source Windows NT clone that aims to be 100% binary compatible with software written for Microsoft's NT-based operating systems (a la Windows XP).
This Windows clone is still in an early Alpha state, but it looks very promising. You can download a copy to try yourself from its official website.
Visiopsys is a GUI-based hobby operating developed from scratch by programmer Andy McLaughlin in his spare time. The result, while limited compared to a commercial operating system, feels impressive for a one-man operation. It's open source and available for free download on the Visiopsys website.
Of all defunct OSes, BeOS may have had the most promise. It emphasized simplicity, elegance, and support for parallel processing. The cancellation of development in 2001 upset BeOS fans.
Enter Haiku, a free, open-source BeOS. Over the past 10 years, Haiku developers have made impressive strides, despite the fact that the OS is still in an early alpha state. Due to Haiku's BeOS compatibility, many quality applications are available for this actively-developed OS. If Haiku continues to improve, the OS could become a mainstream, alternative desktop OS on par with Ubuntu Linux.