Xenomai is a realtime development framework cooperating with the Linux kernel in order to provide pervasive, interface-agnostic, hard realtime support to userspace applications, seamlessly integrated into the GNU/Linux environment. Xenomai is based on an abstract RTOS core, usable for building any kind of realtime interface, over a nucleus which exports a set of generic RTOS services. Any number of RTOS personalities called "skins" can then be built over the nucleus, providing their own specific interface to the applications by using the services of a single generic core to implement it.
Yet Another Machine Simulator (YAMS) is a simple yet realistic machine simulator, which is mainly aimed for teaching purposes on operating systems courses. It simulates one or more MIPS32 CPUs and I/O devices such as disks, terminals, and network cards. Additional devices can be implemented as separate programs that use its I/O plugin interface. The device properties are very configurable. High performance was not a priority factor in its implementation since the main purpose is educational, so YAMS is not an answer for people looking for a fast MIPS emulator/simulator.
Z88Transfer is a transfer utility written with Python and PyGTK that allows a Linux machine to send and receive files from Z88 computers (created in 1987 by Sir Clive Sinclair in his last computer company, Cambridge Computers). Z88transfer supports IMP-EXPORT, EAZYLINK, and PCLINK-II protocols, and can convert automagically between Abiword/RTF format and PipeDream, Z88's native format.
ArcEm is a register level emulator of the Acorn Archimedes A4x0 system based around the ARM microprocessor. It is distributed with ARM Linux binaries and will boot ARM Linux and Acorn's RISC OS 3.1. It runs under X, Win32, and RISC OS. It is based on the release of the ARM emulator core by ARM Ltd, although this has been heavily modified for the emulation. It is a reasonably accurate register level emulation and as such should run pretty much anything.