Clicker - multitasking open-source operating system project, based on a modular micro kernel and a new architecture
Compatible Time-Sharing System (CTSS) - written by a team of MIT programmers led by Prof. Fernando J. Corbato in 1962, immediate ancestor of Multics
CP/CMS - ran on the 360/67 and was also developed in Tech Square
Dartmouth Time-Sharing System (DTSS) - operating system for the GE 635, Honeywell 6600, and DPS8 series of 36-bit mainframes, which was originally developed at Dartmouth in the late 1960s and substantially modified through the 1970s
Darwin - core of Mac OS X, which is based on FreeBSD and Mach
GNU Hird of UNIX-replacing daemons (HURD) - GNU project's replacement for the Unix kernel, collection of servers that run on the Mach microkernel to implement file systems, network protocols, file access control, and other features that are implemented by the Unix kernel or similar kernels (such as Linux)
Mach - operating systems research project of the Carnegie Mellon University School of Computer Science from 1985 to 1994, basis for a number of current projects
Menuet - very compact OSS operating system written in 32-bit x86 assembly code
MINIX - free microkernel UNIX clone that is available with all the source code, created by Andy Tanenbaum
Möbius - 32-bit monolithic kernel design for i486 and higher
ModulOS - OS for the i386 architecture, where each device driver, file system, etc, is a module that can be loaded and unloaded at run-time from the system
OppcOS - an orthogonal persistant OS which means that it preserves its entire system status across system reboots, replacing the concept of file systems for persistancy
OS-9 - preemptive multitasking multiuser capable real-time operating system from Microware Systems, Inc., which is used primarily in embedded systems and industrial controllers but is also used to run personal computers
OS News - online magazine covering alternative operating systems and computing environments
QNX - designed specifically for embedded developers
RDOS - multi-threaded OS for PCs and 386+ based embedded systems, runs many DOS, DPMI and Win32 console programs, 100% assembly with segment protection
ReactOS - OSS effort to develop an operating system that is compatible with Windows NT applications and drivers
real-time systems - list of sites related to real time operating systems and programming