QEMU is a fast processor emulator. Using dynamic translation it achieves a reasonable speed while being easy to port to new host CPUs. In its user mode emulation mode, it can launch Linux processes compiled for one CPU on another CPU. Linux system calls are converted because of endianness and 32/64 bit mismatches. In its full system emulation mode, it emulates a full system, including a processor and various peripherials.
Atari800 is an Atari 8-bit computer (400, 800, and XL and XE series) and Atari 5200 game system emulator for DOS, Windows, Amiga, Atari ST, Mac, and Linux/UNIX. It includes support for Atari cartridge ROMs, popular Atari disk images files, running Atari binaries directly from the host system, and accessing the host filesystem from within the emulated Atari.
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.
xtrs is a Radio Shack TRS-80 Model I/III/4/4P emulator for Unix and the X Window System. It includes lower case, the real time clock, hi-res graphics, serial port, parallel printer, mouse, cassette, sound and music output (requires OSS), 5" and 8" floppy disk drives, and even hard disk drives. The emulated floppy and hard disk file formats are compatible with the popular MSDOS-based emulators by Jeff Vavasour, Matthew Reed, and David Keil. Under Linux, physical floppy disk drives are also supported. Physical cassettes can even be read and written.