Before jumping into my own list, I should say up-front that not all of these systems are dead, or really even obsolete. My first two, OS/2 and NeXTStep, actually live on in evolved versions.
1) OS/2 Warp - Put aside all the fan-boy ranting about how it was the greatest thing since TOPS-20 (and if you get that reference, congratulations you really are a fellow operating system geek), OS/2 really was a great, solid 32-bit operating system, when everything else running on a PC was 16-bit.
Even now, there are parts of OS/2 I'd like to see revived and made open-source. In particular, I think OS/2's SOM (System Object Model) could work well in Linux today.
OS/2, however, isn't dead. It lives on in Serenity System's eComStation desktop. If you want to see for yourself what all the excitement was about, try eComStation. Heck, you may even find yourself becoming a new OS/2 user.
2) NeXTStep - I was lucky enough once to own a Color Turbo NeXTStation. I still wish I had one. To this day, that 33MHz system gave me the smoothest combination of graphics, interface, and hardware I've ever seen on a PC. Mac OS X, the next best thing, is a direct descendant of NeXTStep.
You don't have to have a Mac, a NeXT box, or a copy of the old NeXTStep for 486 installation disks, to see why I liked it so much. The Linux/Unix Window Maker GUI is designed to look and feel like the NeXTStep GUI. If you want to build NeXTStep-style programs, the 21st century version of the application framework, GNUStep, which uses a Cocoa/OpenStep API (application program interface), is still an active open-source project.
3) Interactive Unix SVR3.2 and RVR4 - Long, long before there was Linux, I was using Unix on everything from PCs to mini-computers. My first PC Unix was SCO/Microsoft Xenix on a 4.77MHz 8086 processor. Yes, you read that right; SCO and Microsoft once worked together and release a version of Unix. And, yes, Xenix on that processor was S L O W. The first Unix I really liked on the x86 family though was Interactive Unix back in the late 80s.
It worked well and it was quite fast. Unlike many Unix users, I didn't have any trouble picking up System V's command syntax, so I didn't have the almost allergic reaction that many BSD Unix users had to Interactive and all the other System V Unixes of the late 80s and early 90s. To this day, System V style commands spring to my fingers faster than BSD equivalents.
Sun bought Interactive Unix and that was pretty much the end of it. Sun went on to create Solaris from System V Unix and its own BSD-based SunOS.
4) SunOS 4.1.4 - Speaking of SunOS, I'm one of those cranky people who still liked ye old Sun operating system. It's always seemed to me that while adding System V goodies to SunOS was a fine idea, Solaris ended up having too many odds and ends added to it.
SunOS, which I used back in the day on everything from Sun-3 workstations to SPARCstations, was a very clean and fast operating system. OK, I admit Solaris has a lot of good stuff in it, like Dtrace and ZFS, but I'm sure I'm not the only one that liked the mean, trim, and ready to process SunOS.
5) BSD/OS - The BSD Unix operating systems, such as FreeBSD, OpenBSD, DesktopBSD and NetBSD, are all alive and well, but BSD/OS didn't make it. I always rather liked BSD/OS, which was a commercial BSD, and I'm sorry that it disappeared from the scene.
It was fast, it has commercial support, and, unlike SunOS, it was going stay true to its BSD roots. It wasn't to be though.
In a way, it's because of BSD/OS that we have Linux now. BSD/OS' creator, (BSDi) Berkeley Software Design Inc., ended up in what you could describe as the first of the open-source lawsuits -- AT&T/USL vs. BSDi-in 1992. After Novell bought Unix and AT&T's Unix System Laboratories, Novell dropped the lawsuit. In the meantime, though, little work was done with any of the BSDs. And, so a Finnish college student named Linus Torvalds decided to build his own Unix-like operating system. And, the rest as they say, is history.