Emulator

An Emulator, in the most general sense, duplicates (provide an emulation of) the functionality of one system on a different system by translating calls designed for the target hardware into calls that the host system's software can understand and interpret correctly into an output, so that the software appears to behave identical to the target system. Unlike a simulation, it does not attempt to precisely model the state of the device being emulated; it only attempts to reproduce its behavior. This behavior can have differing degrees of accuracy between emulators.

A popular use of emulators is to run software and games, often referred to as ROMs, written for hardware that is no longer sold or readily available, such as the Commodore 64 or early Amiga models. Emulating these on modern desktop computers is usually less cumbersome than relying on the original machine, which may be inoperable. However, software licensing issues may require emulator authors to write original software that duplicates the functionality of the original computer's bootstrap ROM and/or BIOS, often through high-level emulation.

Legality of emulators
Most emulators are perfectly legal under United States and international law, protected by laws that cover reverse engineering. However, emulators can be illegal if they use copyrighted code from the original console, computer, or program.

Legality of ROM images
ROM images are copyrighted code and are protected by international law. The only legal images are homebrew games of original content, created by programmers, images released into public domain, or images downloaded with the permission of the copyright holder.

Common legality myths

 * 24-hour rule: Many emulation download sites like to claim that you can keep unlawfully downloaded ROMs for 24 hours and then delete them, and still remain legally compliant with the law. This rule has no basis in fact.
 * Owning the game: Owning the game in its original form does not make downloading a ROM image of it legal, according to the letter of the law. The only fully legal way is to make a backup image yourself using special hardware. However, this hardware can be expensive and/or complex to create (and illegal sometimes, as is the case of Nintendo ROM image-extracting hardware), although many gamers consider it ethically correct to download an image as long as you own the original game.

Arcade

 * MAME

PlayStation

 * Bleem!
 * ePSXe
 * PSXeven
 * AndriPSX
 * PCSX (No Bios)
 * PCSX-Reloaded
 * SSSPSX
 * Psxjin
 * NO$PSX (No Bios)
 * hpsx64 (PS1 and PS2) (64-bit)
 * Mednafen

PlayStation 2

 * PCSX2
 * NeutrinoSX2
 * PS2emu
 * Play!
 * hpsx64 (PS1 and PS2) (64-bit)

PlayStation 3

 * RPCS3

PlayStation Portable

 * PCSP
 * Jpcsp
 * Potemkin
 * PSPE
 * PPSSPP

Super Nintendo Entertainment System

 * ZSNES
 * Snes9x
 * Mednafen
 * Unsnes
 * SNESGT

Nintendo Entertainment System

 * NESticle
 * VirtuaNES
 * NEStopia
 * JNES
 * FCEUX
 * FCE Ultra
 * Famtasia
 * Mednafen

Xbox

 * Cxbx
 * Cxbx-Reloaded
 * Dxbx
 * IronBabel
 * Xenoborg
 * Xeon
 * XProject
 * xqemu
 * Hackbox
 * XbeNext

Xbox 360

 * Xenia
 * ex360e (An Experimental Xbox 360 Emulator)
 * exbox360 (Also an XBOX 360 emulator in development)

Virtual Boy

 * Mednafen

Nintendo 64

 * Project 64
 * 1964
 * UltraHLE
 * Nemu64
 * Mupen64
 * Corn
 * SuperHLE
 * Daedalus
 * UltraHLE 2064
 * TR64
 * Apollo
 * TRWin

GameCube

 * Dolphin
 * Dolwin
 * GCEmu
 * WhineCube
 * Gcube
 * Gekko

Wii

 * Dolphin

Wii U

 * Cemu

Dreamcast

 * NullDC
 * Chankast

Saturn

 * SSF (No Bios)
 * Satourne
 * Saturnin
 * Yabause
 * Gens32

Mega Drive/SEGA CD

 * Genecyst
 * KEGA
 * Kega Fusion

PC-Engine/TurboGrafx-16

 * Magic Engine (No Bios)
 * Ootake
 * PCEjin (No Bios)
 * Mednafen
 * MESS

PC-FX

 * Magic Engine FX (No Bios)

3DO

 * 4DO
 * FreeDO

Game Boy

 * VisualBoyAdvance
 * VisualBoyAdvance-M

Game Boy Color

 * VisualBoyAdvance
 * VisualBoyAdvance-M

Game Boy Advance

 * NO$GBA
 * VisualBoyAdvance
 * VisualBoyAdvance-M

Nintendo DS

 * DeSmuME
 * No$GBA
 * WinDS

Nintendo 3DS

 * Citra
 * 3dmoo

PlayStation Vita

 * Vita3K

Personal computer emulators

 * DOSBox
 * MESS
 * QEMU
 * Bochs

MSX

 * blueMSX
 * OpenMSX
 * RuMSX

NEC PC-8801

 * M88

NEC PC-9801

 * Anex86
 * Neko Project II
 * T98-Next

FM TOWNS

 * Unzu

X68000

 * XM6