Started in 1996 and still in beta, SkyOS is primarily developed by just one person, Robert Szeleney. This proprietary OS has a lot of good things going for it, such as symmetric multiprocessing support, an integrated media subsystem, and a journaled 64-bit file system that lets you recover a partition in the event of a crash. Application support is limited, but SkyOS offers Mozilla Firefox and Thunderbird, AbiWord, Gaim, Nvu, and Pixel, among others. Installing application software is simple via the SkyOS Software Store; all it takes is a few clicks of the mouse.
SkyOS also offers real-time file content query searches with multiple keyword support, similar to Beagle in Linux or Spotlight in Mac OS X. Files and programs are indexed for easy access. Also included is support for system-wide mouse gestures, and a media player application that uses ffmpeg for multiple codec support, including Windows Media Video and MP3.
On the negative side, SkyOS hardware support is extremely limited (though by running SkyOS through VMware you can get the graphics card, sound, and networking support of your native OS). Only a handful of network cards and sound cards work, and only a few graphics cards allow for full 2-D acceleration.
If you have hardware that's supported, you can pay the project $30 to join the beta program. If, however, you prefer a stable operating system, wait until SkyOS releases its first official version.
After Be Inc.'s assets were bought Palm in 2001, a group of BeOS fans created the OpenBeOS project to write an open source operating system (under the MIT license) that would be backward-compatible with BeOS 5. The project, now known as Haiku, has made some progress since it started. A lot of programs can run on the system, including Mozilla Firefox and SeaMonkey, as well as some games and other programs.
While Haiku offers short boot times and application load times, networking adapter support is very limited. Also, installing Haiku requires a BeOS or Zeta system installed, and an extra BeOS partition to move the Haiku files onto.
Syllable, created in 2002 as a fork of the now-defunct AtheOS, is a GPL-licensed open source desktop OS. It has modest hardware requirements and boots quickly. Although the choice of application software is limited, there are Web browsers, email programs, games, and a media player. To install the majority of the software, you just move a binary file out of an archive. Syllable also includes a 64-bit file system, the AtheOS File System.
Syllable's hardware support can be lacking, and installing Syllable can be an adventure for some, as the installer is text-based and can be confusing. However, out of all the operating systems in this list, this is the most ready to be used as a secondary OS.
Visopsys, the Visual Operating System, is an open source OS licensed under the GPL and LGPL that was started in 1997. However, if you're thinking of running Visopsys as a desktop OS, you might think again, as this is more of a hobby OS developed primarily by Andy McLaughlin. Visopsys offers fewer applications and much more restricted hardware support than the others in this list.
While Visopsys may not be the best desktop OS, it has one great use. Partition Logic, a free, open source partition manager like Symantec's Partition Magic, uses Visopsys, and in fact was created by the same person. Partition Logic works just as well as Partition Magic, and makes for a great, free alternative.
So there you have four alternative operating systems in development. While they may not have the best hardware support out there, and may not be the easiest to install and use, their ideas and designs set them apart. In a world where Linux, Windows, and Mac OS X have control of public mindset of the operating system, it is nice that other OSes can make strides and try different things.