End of an era: Mac OS 9
From the first Apple Macintosh's release back in 1984, the Mac OS (also known as the System, as in System 6, System 7 and so forth) was constantly improved, tweaked but not overhauled - an evolution of the ground-breaking operating system that first shipped.
This classic Mac OS had many fans, despite its many shortcomings; a lack of pre-emptive multitasking or proper protected memory made it notorious for potential instability, extensions could easily conflict, and the once unique GUI had long since been aped (some would say bettered) by Microsoft Windows.
The development of Mac OS X was to finally push the classic Mac OS aside, the result of entirely too many years' development at Apple into a replacement for the aging system. Built as an entirely new OS, with BSD and the Mach kernel at its core, Mac OS X promised much that the classic OS could not, and has now superceded it as Apple's flagship operating system.
But before its demise, the classic Mac OS had one final swansong in Mac OS 9. Billed by Apple as "the best Internet OS", it was to lead the Macintosh into the 21st Century on a high, the very peak of the original Macintosh system's evolution.
"In The Beginning": Birth
Mac OS 9 started life intended to be the next point release of Mac OS 8, designated as Mac OS 8.7 and sporting the code-name "Sonata". It soon was renamed Mac OS 9, however, a change which prompted a lawsuit by Microware (whose OS-9 operating system, they believed, would have its name diluted by Apple's offering).
Mac OS 9 saw launch on October 23, 1999, and was the second major operating system release from Apple that was PowerPC-only, the first being Mac OS 8.5. Every previous Mac OS right up to Mac OS 8.1 had been available for the Motorola 68k-based Macs, but from 8.5 those ties were cut. The nanokernel that had seen introduction in Mac OS 8.6 was retained, as was OS 8's 'Platinum' appearance; indeed, Mac OS 9 always looked rather similar to OS 8, cosmetically, barring details like the version number on the splash screen or the "About This Macintosh" dialog box.
As well as updates to the operating system itself, Mac OS 9 shipped with many new features. Sherlock 2 was advertised on the box, and boasted greatly improved search capabilities both for on- and offline searches. The iTools, free services such as the online "iDisk" storage facility, were also available to buyers of OS 9 - these would later be rebadged as .Mac, and became a subscription-based service. Also notable was the inclusion of an online Software Update control panel, providing a Windows Update-alike for the Mac OS.
One long-awaited feature in the Mac OS finally arrived, as support for multiple users was introduced via another control panel. Like other multi-user systems, each user had a unique username and a password, but the Mac OS's implementation also allowed the user to use a "speech password", that could be spoken to log in. This interesting feature, sadly, was not incredibly successful, as it proved difficult to recreate the voice password accurately enough to log in each time. The Keychain was another new feature that carried over to Mac OS X; one could store all their passwords in the keychain, and lock or unlock it to control access to all those items. (mkb informs me the Keychain actually saw its debut in System 7 Pro, as part of the poorly-received "PowerTalk" tools; it was, however, trumpeted by Apple as one of the key new features of Mac OS 9.)
Whilst Mac OS 9 will install on just about any PowerPC-based Macintosh released by Apple, it is not officially certified for use on the Mac clones due to alleged compatibility problems. While it can be installed, it is not recommended by Apple.
Mac OS 9 saw updates from 9.0.2 through to 9.0.4, before the release in 2001 of Mac OS 9.1.
Onward and Upward: Adolescence
2001's Mac OS 9.1 was a free update to owners of Mac OS 9, and included yet more features to the rapidly maturing Mac OS. Most notable of these was the inclusion of CD writing support to the Finder.
Mac OS 9.1 also added the "Window" menu to the Finder's menubar, complimenting the application switcher with a means of cycling through and selecting a visible window without having to organise them on the desktop. Mac OS 9.1 also proved to be more stable than 9.0.4.
Mac OS 9.1 is the last version of Mac OS 9 that many Macintoshes can support. While not the final release of the classic Mac OS, the following release will only install on a Mac capable of running Mac OS X - essentially a Power Macintosh G3 or better. This leaves lesser PowerPC-based Macs without a means to upgrade to the even more stable 9.2 - at least, officially. Workarounds such as OS 9 Helper (available here) can be used to convince the installer to run on an "Old World" Mac.
The Decline and Fall: Old Age and Death
Leaving the Old World Macs behind, Mac OS 9.2.1 saw release later in 2001. The main purpose of 9.2 was to improve support for the "Classic Environment", which allows for classic Mac OS applications to run within Mac OS X. However, many feel the improvements included in 9.2.1 and its follow-up, 9.2.2, to be too good to be just for New World Macs.
Whilst intended solely to improve support for Classic in Mac OS X, 9.2 can be booted as a stand-alone operating system on any Macintosh that supports booting into Mac OS 9. This includes any Power Macintosh G4 except the second-to-last revision (the very last Power Mac G4s were enabled to boot into Mac OS 9 again), or any Mac released before them.
With the release of Mac OS 9.2.2, the long legacy of the classic Mac OS comes to a close. While there are those who still run Mac OS 9 as their primary OS, due to aging hardware or personal preference, the operating system is now officially dead according to Apple; head honcho Steve Jobs even held a funeral for it at the WWDC in May 2002. While still distributed as the Classic Environment for Mac OS X, this too is set to finish, as Apple introduce Intel-based Macintoshes with no support for the PPC classic OS or its applications.