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.
BizHawk is a an emulator for multiple game system home computers. It's written in C#, provides full-screen viewing, joypad and joystick support, as well as development and debugging features for all emulated CPUs and hardware platforms. It supports NES, SNES, N64, Gameboy/Color/Advance, Sega Master System, Genesis, Saturn, SG-1000, Game Gear, PC-Engine (TGfx-16), Atari 2600/7200, SuperGrafx, ColecoVision, TI-82, Wonderswan, PSP, PSX, and C64.
VICE is a program that runs on a Unix, DOS, Win32, OS/2, BeOS, Amiga or Mac OS X machine and executes programs intended for the old 8-bit computers. The current version emulates the C64, the C64DTV, the C128, the VIC20, practically all PET models, the PLUS4 and the CBM-II (aka C610/C510). An extra emulator is provided for C64 expanded with the CMD SuperCPU. VICE is *free* software released under the GNU General Public License, and as such it comes with full source code.
Bochs is a powerful x86/amd64 hardware emulation and debugger software. Besides the CPU and instruct set extensions, it simulates common PC/AT hardware, an ISA and PCI system bus, memory, disks, video hardware, ethernet, and enough of a compatible BIOS to run DOS/Winows, Linux/BSD, Minix and other operating systems. It doesn't provide virtualization, thus runs extremely slow, but is safe to run for the host operating system.
libtmt is the Tiny Mock Terminal Library. It provides emulation of a classic smart text terminal, by maintaining an in-memory screen image. Sending text and command sequences to libtmt causes it to update this in-memory image, which can then be examined and rendered however the user sees fit. libtmt is similar in purpose to libtsm, but considerably smaller (500 lines versus 6500 lines). libtmt is also, in this author's humble opinion, considerably easier to use.
x3270 is an IBM 3270 terminal emulator for the X Window System and Windows. It runs on most Unix-like operating systems -- e.g., Linux, Mac OS X, Solaris and Cygwin. It also runs natively on Windows. x3270 runs over a TELNET connection, emulating either an IBM 3279 (color) or 3278 (monochrome). It supports: The full TN3270E protocol, SSL/TLS (via the OpenSSL library) for encrypted sessions, APL2 characters, non-English character sets, including Russian, Turkish, Hebrew and DBCS Chinese and Japa
QEMU is a generic machine emulator and virtualizer. It can emulate other CPU and host architectures, or run native binaries with native performances by executing the guest code directly on the host CPU any only emulate devices. It's thus often used in conjunction with KVM on Linux for system virtualization.
vDos is a DOSBox fork which omits some graphics and gaming emulation in favor of supporting old DOS text-mode and business applications. It adds a live filesystem bridge, file and record locking, Windows printing, scalable screen fonts, and other features. It's rather convenient to set up, runs on Linux, Mac OS X, and Windows systems.
A fork of the Gngeo arcade emulator that fixes some things broken in version 0.8, and adds some new features.
Mono is an open source implementation of Microsofts .NET Framework, which in turn is meanwhile based on the ECMA standards for C# and the Common Language Runtime. Mono provides its own C# compiler, and a JIT/AOT-based VM. It adds true runtime cross-platform support by providing Gtk/Cairo, OpenGL, LDAP, and POSIX bindings, and further integration with Wine. Mono runs on Linux, BSD systems, MacOS, PS3, Wii, Android, x86/amd64, PowerPC, Sparc, ARM, Alpha systems. Besides C# 4.0-6.0 support, it can
MEWUI is a U.M.E. (MAME/MESS) user interface frontend. It extends the catalog browser, adds the full list of games, allows filtering, shows snapshots and cabinets in the selection menu, and adds an in-game mameinfo menu.
Zilog Z80 is a C++ shared library and frontend for emulating the like named home computer system. It simulates all hardware pins of the Z80 chip (which was binary compatible to the i8080), and comes with a CP/M 2.2. emulator accessible vis telnet client. It alsp provides an disassembler and instruction set examiner, a ZX-80 and TK-80 emulator.