OS-9
Shame on you for not mentioning OS-9, the Unix-like OS created by Microware Systems Corporation to support the vision of the creators of the Motorola 6809, the best 8-bit microprocessor ever (unless you count its second-sourced offspring, the Hitachi 6309). OS-9 dates back to 1980 or a little before, and could multitask with 64K of RAM. It ran on many 6809-based systems, and later on moved to 68K family processors and then to many others in a version rewritten in C, but it is probably best known as the operating system of choice for the Tandy Color Computer in its three incarnations.
In the early '80s I heard of this Unix-like OS called OS-9 that ran on the 6809 processor that the CoCo [TRS-80 Color Computer] used. It was great -- could multitask up to 16 programs in 64K smoothly but slowly, given that the CPU ran at 1.8 MHz. And it had an optional GUI layer that even sported a WYSIWYG word processor. Before the Mac. 10 years before Windows. In 1989 I bought my first IBM-style PC and was stunned at how primitive it was by comparison, especially given the price.
I loved OS-9 on the Tandy Color Computer 3. Had 640KB memory, the dual double-sided 3.5-in. drives and a whopping 20MB hard drive with OS-9 loaded on it. We ran a Bulletin Board System on that for almost 5 years before the Internet was even around. Long live FIDO Net.
RISC OS
U.K. 1987ish: Acorn's OS evolution from the BBC Micro to its new ARM RISC-based hardware -- ahead of its time in terms of usability IMHO, with "application directories" and a very consistent drag-and-drop that was a delight to use, even when compared to what we've got today. On the original ARM hardware, it also gives really easy access to ARM assembly language via a built-in assembler within standard BBC BASIC -- great for anyone wanting to learn about the ARM CPU!
In fact, RISC OS is actually still around today if you look hard enough. It can be emulated on x86 or OS X ... and was even open-sourced recently. It still has a small, active and very friendly community. Well worth a try for anyone interested in neat, fast user interfaces. Some good Web starting points are "RISC OS Ltd.", "RISC OS Open" and "Drobe." With the forthcoming ARM (Ubuntu) based netbooks (June 2009ish), I might even end up back where I started!
Acorn's (not quite dead) RISC OS could still teach GUI designers a thing or two about ease of use, properly anti-aliased and hinted display fonts as well as consistent user interface -- especially where drag-and-drop is concerned.
OASIS
In 1980 I went to work for a company as a product manager and found our product based on an operating system called OASIS [Online Application System Interactive Software]. I had been working with CP/M at the time and this blew me away. Looked like Pick on a PC. Acted like Pick on a PC (actually, since this was before the PC was invented, we called them microcomputers).
OASIS offered multitasking, multiuser support on a Z-80, later ported to the 8080, 8086 and 80286. Like Pick it had a very powerful Basic, a relational database and lots of utilities. I really learned to love that operating system.
OASIS was far ahead of its time and vastly superior to anything that Digital Research or Microsoft had to offer. In fact, before IBM went to DR to ask about CP/M for its first PC, it came to Phase One Systems to ask for OASIS -- if, and only if, its author would be willing to denature it so that it could never, ever be multiuser. Tim Williams was not so willing, and so IBM went on to Monterey and then to Seattle. Oasis-16 ended up on the PC anyway, to the horror of the blue-suited brigade who encountered it at Comdex in Atlantic City.
Mac OS "classic"
Mac OS classic is my favorite -- many a late night playing Dark Castle on a 9-in. black & white screen (esp. waiting until Christmas Day to crack open the Easter egg in the main hallway of that game), throwing rocks at bats & mice, picking up elixir, and (eventually) defeating the beer-chugging Dark Knight. With the only Macintosh (as they were once called -- it's just "Mac" nowadays) in my neighborhood, I was the envy of my childhood peers.
Mac OS classic is alive and well. We still have a couple of older G3 Macs running OS 9. For most of what my wife does (e-mail, AppleWorks and browsing), these machines are still pulling their weight. The only problem is that more and more sites are embedding Flash and it's starting to break for some sites.