I think IRIX hasnt been named yet.
Have you ever used it?
SGI workstations and servers were always very expensive. Plus SGI kept changing their apparent strategy even more often than Sun in the last ten years. You never knew if they wanted you to buy MIPS or Itanium; IRIX, Linux, or Windows NT.
A few years ago I found that an IRIX workstation was in fact ten times more expensive than a comparable Sun workstation.
The chemistry department in my university had SGI workstations in their labs. The maths department (where I was) used Sun workstations. One professor in the chemistry department actually used a surviving VAX running an old version of VMS.
Apart from the odd session in the chemistry computer lab I have never used IRIX. I have also never used VMS.
A few other Unices I have never touched are AIX (IBM UNIX), HP-UX, DEC/Compaq/HP Tru64 UNIX, and some of the more obscure BSD distributions.
I was also an Atari user for a while. Newest version of the OS I have is Multi-TOS/N.AES from 1998 (on CD-ROM). Multi-TOS could multitask and had some support for POSIX APIs (i.e. could run a few UNIX programs including X11R_5_ if you had a 68030 CPU or better).
Don't forget Java and .NET. While they are not operating systems from below (i.e. they don't run hardware), they look like operating systems from the point of view of applications. Likewise, but without the virtual machine, this applies to Cocoa (i.e. OpenStep).
And then there is Microsoft Hyper-V and VMware, which look like operating systems from the hardware's point of view but not from applications' point of view. I use VMware to run Windows Vista 64 on my iMac. And I use Hyper-V at work. (It's very good.)