Most people don't think of them as computer systems, but late-90s and early-2000s phone and PDA OSes. Specifically, EPOC32/Symbian, Apple Newton, BlackberryOS, PalmOS, Qualcomm RexOS, and Windows CE/PocketPC/Mobile (and its later incarnation Windows Phone/10 Mobile)<p>Newer calculators, like the TI-Nspire Cx-series and HP Prisms.<p>Proprietary non-linear editing systems from the 80s and 90s before the switchover to software.<p>A few legacy systems that haven't quite been emulated yet are BeOS, and RISCOS, certain proprietary Unix implementations like A/UX, Amix, Irix, NeXtOS, Apollo DomainOS, AIX, PA-RISC, Ultrix, etc., and Japanese PCs like the PC-98, PC-88, Sharp X1, Sharp X6800, and FM-Towns<p>Minicomputer hardware, like VAX Machines, DEC Alpha Systems, and DECstations, are also lacking in good emulation, but the OSes (OpenVMS and Ultrix) are available to hobbyists.<p>Itanium has not so far been emulated, so IA-64 versions of Windows are probably going to be lost (as no more IA-64 processors are being made) should nothing be done.