Wine is an implementation of the Windows API on top of X and Unix. It does not require Microsoft Windows, but can use native Windows DLLs if they are available. It provides both a development toolkit for porting Windows source code to Unix as well as a program loader, allowing many unmodified Windows programs to run on x86-based Unixes.
XRoar is a cross-platform emulator for the Dragon 32, Dragon 64, and Tandy CoCo computers, all originally released in the early '80s. The emulator runs under Unix-like operating systems (including Mac OS X) and Windows, with older versions available for handhelds like the Nintendo DS and GP32.
spim is a self-contained software simulator for running R2000/R3000 assembly language programs. It reads and can immediately execute files containing assembly language code. spim also provides a debugger and simple set of operating system services. spim provides both a simple, textual interface and a fancier, graphical interface. The package includes complete source code and documentation.
plus4emu is a portable emulator of the Commodore Plus/4 computer. It supports Windows and POSIX platforms (32 bit Windows, 32 and 64 bit Linux, and Mac OS X have been tested). It implements accurate, high quality hardware emulation, but the system requirements are higher than that of most other emulators.
uBee512 emulates all of the Microbee Z80 series of microcomputers, including ROM, Floppy and Hard disk-based models. Up to 2MB of extended memory is supported. The optional on board sn76489 sound IC is also emulated. The display may use SDL or OpenGL video rendering. Z80 PIO emulation includes tape, speaker, RTC, serial to PC RS232, and mouse peripherals. Parallel port peripherals include BeeThoven (AY-3-8910), BeeTalker (SP0256-AL2), Compumuse (SN76489), DAC, printer, and joystick. Many disk image types and floppy disks are supported. A joystick may be optionally mapped to keys. A large number of options are available as well as a built in OSD console.
MDK (MIX Development Kit) provides tools for developing and executing, in a MIX virtual machine, MIXAL programs. The MIX is Donald Knuth's mythical computer, described in the first volume of The Art of Computer Programming, which is programmed using MIXAL, the MIX assembly language. MDK includes a MIXAL assembler (mixasm), a MIX virtual machine (mixvm) with a command line interface, a Guile-based virtual machine (mixguile), a GTK+ based GUI (gmixvm), and a mixvm-Emacs interface (mixvm.el). MDK utilities are extensible using Scheme.