The first kind of computer I used, back in high school, was the TRS-80 Model I. Emulators are now available to emulate TRS-80 stuff, probably at vastly higher speeds, atop Unix-like systems. You may find little point to this, though, as you're limited to low-rez, 64x16 screens, and single case text...
Older computing systems, many of which would be better forgotten...
The progenitor of most of the modern operating systems, most particularly including Unix. Multics introduced a whole lot of ideas to computing that are still being absorbed into the "mainstream."
Kind of like X, only with less network capabilities, and rather lower memory overhead.
The "April 2000" attempt to re-release AmigaOS.
Humorous articles about the Commodore Amiga
No, OS/2 is not moribund. But sales aren't exactly increasing...
Compute! published articles and programs for various 8 and 16 bit platforms, including Apple ][, Atari computers (8 bit and ST), Amiga, Commodore (VIC-20, C=64), TI94 and IBM PC.
Runs, under emulation a PDP-8 or PDP-11. There are disk images of Unix Systems 5, 6, and 7, as well as some DEC and DG OSes. Pretty cool.
It's not really obsolete, just somewhat disused. Kermit was the "original" program that provided substantial communications crosscompatibility between PCs, Minicomputers, and Mainframes.
The software is freely available in source code form from Columbia University; you are merely expected to purchase the Kermit book from Columbia; you must redistribute it with a copy of the book. As a result, Linux distributions (and frankly virtually anything else) are not permitted to include Kermit.
Originally, the Kermit protocols were noted for being fairly slow, but nonetheless fairly reliable, and (critically) often the only choice for communicating files between obscure platforms. It also has a fairly nice scripting language to automate file transmissions.
More recently, enhancements have been made to substantially improve transmission speeds, thus obviating the performance issues. Unfortunately, with the growth of the Internet, with corresponding protocols such as FTP, there has been less need for Kermit, and with its restrictive licensing, it has grown decreasingly relevant.
Kermit is now redistributable with "Certified" Open Source Operating System Distributions, where "Open Source" is defined according to the Open Source Definition. This will certainly increase its proliferation; it may be too late for it to become truly popular...
This paper describes the "PCLSRing" feature of the Incompatible Time Sharing (ITS) operating system. PCLSRing permits a process to access the state of another process in a modular fashion.
Subject of the novel, "The Soul of a New Machine," this was one of the earliest 16 bit minicomputers.
Simulogics now offers emulators that likely run orders of magnitude faster on modern hardware...
See also the The Jupiter Ace hardware page that describes how you might build an Ace-compatible machine yourself, complete with Z-80 and 5K of memory.
The BBN Butterfly was a "massively parallel" supercomputer based on commodity Motorola CPUs, including 68020 and 88100s. In its latter days, it ran a Mach variation. One of quite a number of Dead Supercomputer Projects.
The "PC532" was a build-it-yourself kit computer using the NSC32532 processor (one of the last "formally CISC" processors designed before everyone went nuts about RISC). People have run NetBSD, Mach, and Minix on it, and as a "home-built," it is quite likely more publicly documented than any other computer.
Including manuals for Fortran, Algol, Pascal, Modula-2, Modula-3, PL360, Bliss, BCPL, C, ML/1, flowcharts, and some early computing patents
Technical manuals for a LOT of ancient computer systems, such as PDP-1 thru PDP-15, Illiac, Interdata, CDC, Burroughs, PERQ, Prime, Ridge, SDS, TI Explorer, Univac, Xerox 820, Alto, Dorado, Apple 1, and others...
The Contiki desktop environment is a highly portable, modern, open source, Internet-enabled operating system and desktop environment for very constrained systems, such as 8-bit home computers.
Written in C, ported to various 8 bit systems such as C64, Atari 800 series, TRS-80 CoCo, and various video game systems.
Including early GUI machines, Ethernet, CP/M, and Lisp machines.
For aficionados of the 6800/6809 OS. UniFLEX was the last generation of the TSC FLEX Operating Systems, and was a true multi-user unix-like operating system running on the 8/16 bit 6809 microprocessor.
System09 is intruction set compatible with the 6809 and is intended to run the Flex 9 operating system. It includes a 2KByte SBug monitor program along with a UART and simple timer.
Written for the Intel 8080 in PL/M, this was the first really popular microcomputer operating system.
ZSDOS - a CP/M successor now available under the GPL.
This was technically a much better OS than MS-DOS, ten years ago, despite being based on the Z80 rather than the more powerful x86 lineup of processors...
Extracts files from the CP/M LBR format, on Linux -like platforms
A CP/M emulator for Linux. It does not emulate the entire machine, but instead emulates a Z80 CPU, and captures BDOS and BIOS calls to execute them as the hosting OS. As a result no disk image is needed; the system may access any file in the directory that matches CP/M file naming conventions.
There's the potential for an enormous performance boost out of this; file access can map pretty directly from BDOS/BIOS calls onto LIBC requests, instead of having to go through a further emulation process whereby requests are processed through an "emulated" CP/M filesystem.
Archiver for CP/M, written in portable C to run on various Unixes.
A C compiler widely used on CP/M.