A computer's operating system and software form a complex, multi-tiered ecosystem where modules, applications, drivers, services, and other system components interact with each other on various levels of abstraction (A fancy stack). Older, out of date OS's are not listed below.

Microsoft Windows XP - Windows XP has ignited a fury of upgrades, debates, and questions. Many of us are gazing into a future of a truly unified driver model for Microsoft's OSes. There are now both 32 bit and 64 bit packages available.

Windows XP is an advance of the codebase from Windows 2000, Windows NT, and the MS/IBM project before that. After the break-up, Microsoft kept the name NT (New Technology) while IBM used the name OS2. The NTFS file structure was created primarilly by the SCO team hired by IBM and based on SCO UNIX with many added features like better garbage cleanup and name authentification.

http://www.microsoft.com/windowsxp

Linux - The Debian Project is an association of individuals who have made common cause to create a free operating system. This operating system that they have created is called Debian GNU/Linux, or simply Debian for short. Debian systems currently use the Linux kernel. Linux is a completely free piece of software where the kernel was created by Linus Torvalds, the OS is GNU, and is supported by thousands of programmers worldwide. LInux is 32 bit.

Many companies like Redhat, Susie and Mandrake distribute GNU operating systems, but Debian is GNU and is free for the download.

Linux is pronounced Lin-niks and stands for Linus's Little Minix; that's a short "I" not long. Here's Linus Torvald pronouncing Linux.

http://www.computer.org

Mac OS X is Apple's new operating system. I've said it before and I'll say it again: the "X" is pronounced "ten", like the roman number, not "ex" like the letter. Don't make me come over there and whup you.

Macintosh's new 32 bit OS is a combination of a Mach kernel from Carnegie Mellon University, a BSD UNIX operating system from University of California-Berkley, and Apple created GUI which is loosely based on the "NEXT" GUI work.

http://www.apple.com/macosx

http://www.sgi.com/software/irix6.5

Sun Solaris® - Customers can take full advantage of UltraSPARC® processor-based systems, from smaller departmental servers to massive SunPlex® clusters with hundreds of CPUs. Designed for multiprocessing and 64-bit computing, Solaris software scales to handle heavy traffic, huge data sets, and compute-intensive problems.

http://wwws.sun.com/software/solaris

FreeBSD is an advanced 32 bit operating system for x86 compatible, DEC Alpha, and PC-98 architectures. It is derived from BSD UNIX, the version of UNIX developed at the University of California, Berkeley.

http://www.freebsd.org

AIX 5L and its success as the native platform for IBM pSeries continues to attract extensive support from the UNIX industry’s most successful application vendors. In addition, the affinity features of AIX with Linux provide customers the flexibility to leverage AIX and Linux together where appropriate, helping to preserve investments in skills and applications.

http://www-1.ibm.com/servers/aix

SCO open UNIX® 8 Release 8.0 (UnixWare® 7 inside, Release 7.1.2) is the most advanced deployment platform for industry standard Intel and AMD processor systems. Open UNIX 8 is the trusted foundation for solutions where proven scalability, reliability and affordability are critical.

http://www.caldera.com/products/openunix

New information: SCO, using invalid claims, is attempting to hijack Linux from the open source world by sueing IBM and any company using Linux. My advise would be to not use SCO or Caldera products. This seems to be a last resort attempt of a failing company. IBM intends to fight this, and let's hope they crush this foolishness along with SCO.

The idea that Linux can be owned was most eloquently explained by Eric Raymond, the president of the Open Source Initiative, in a recent position paper he wrote on the SCO case.

Who owns Unix, anyway? http://www.opensource.org/sco-vs-ibm.html#id2854348