Nice piece in Computerworld on ten operating systems the world left behind:
10 OSs: Gone but not forgotten
CP/M
DOS
Mac OS
Amiga OS
GEOS
OS/2
NeXTStep
BeOS
Windows 95
Forgotten but not gone: The X Window System
I wonder which modern OS will disappear before the others? Windows? Unlikely. The one I think will continue to grow is Mac OSX (both regular and iPhone versions).