Emulation Software

An emulator mimics (obsolete) hardware (i.e. electronic components) in order to run an (obsolete) Operating System.

This is different from Virtualization where there is no (or minimal) emulation of hardware performed. Virtualization software uses hardware of the machine it runs on and acts as a gateway to share resources between the host-machine and the virtualized Operating System.

Amiga

 * Amiga Forever (commercial, Amiga, website)
 * Fellow (freeware, Amiga, website)
 * UAE (open source, Amiga, website of WinUAE website of FS-UAE)

Amstrad CPC

 * Arnimedes (freeware, CPC, website)
 * Arnold (open source, CPC/CPC+/KC compact, website)
 * CPC++ (shareware, CPC, website)
 * CPCE (freeware, CPC, website)
 * CPCEmu (open source, CPC, website)
 * WinAPE (freeware, CPC, website)

APF Imagination Machine

 * APF_EMUW (freeware, Windows, website)

Apollo Guidance Computer

 * Moonjs (in-browser simulator, website)

Apple II

 * AppleWin (open source, Apple IIe, website)
 * KEGS (open source, Apple IIgs, website)
 * OpenEmulator (open source, Apple II, website)

Apple disk image that detects which emulator you're using to run it

Arcade

 * MAME (open source, Arcade, website)
 * Raine (open source, Arcade, website)
 * Retrocopy (freeware, Arcade, website)

Atari
See also ROM and memory images.
 * Atari800 (open source, Atari 800/800XL/130XE/5200, website)
 * Altirra (open source, Atari 800, website)
 * EMU7800 (open source, Atari 2600/7800, website)
 * EstyJS (open source, Atari ST, JavaScript, website)
 * Hatari (open source, Atari ST/STE/TT, website)
 * Javatari (open source, Atari 2600, website)
 * ProSystem Emulator (open source, Atari 7800, website)
 * Stella (donationware, Atari 2600, website)

C64

 * CCS64 (shareware, C64, website)
 * Commodore 64 for iOS (paid download)
 * Frodo (open source, C64, website)
 * VICE (open source, C64/C128/Plus4/VC20/PET/..., website)

Coleco ADAM

 * ADAMem (freeware, ADAM, website)

Digital Group

 * Digital Group emulator (Mac, Windows)

Firefox OS

 * Firefox OS Simulator (open source, Windows/Mac/Linux) announcement

Macintosh

 * Basilisk II (open source, 68000, website)
 * Mini vMac (open source, Macintosh, website)
 * PearPC (open source, PowerPC, website)
 * SheepShaver (open source, MacOS, website)
 * vMac (open source, Macintosh, website)

Next

 * Previous (open source, website)

Nintendo
See also ROM and memory images.
 * higan (formerly known as bsnes) (open source, primarily SNES, but recently also supports NES/GB/GBC and GBA, website)

Sega
See also ROM and memory images.
 * Genecyst (freeware, Genesis/Mega-Drive, website)
 * Gens32 (freeware, Genesis/Mega-Drive/Sega-CD/Mega-CD/32X, website)

Sol

 * Solace (freeware, SOL-20, website)

TRS-80

 * Various emulators here
 * Color Computer Emulator here

x86

 * Dioscuri (open source, x86, website)
 * DOSBox (open source, x86, website)
 * DOSBox-X
 * DOSEMU (open source, x86, website)
 * iDOS (DOS emulator for iOS)
 * JPC (open source, x86, website)
 * PCE (open source, x86, focus on old PC/XT/AT systems, website, JavaScript port)

ZX Spectrum

 * Fuse (open source, ZX Spectrum, website)

Other and Multiplatform

 * Daphne (open source, LaserDisc, website)
 * EaaS (emulation-as-a-service; see in-browser sandbox )
 * GXemul (open source, generic, website)
 * MESS (source available, multiple home computers and other computers, website)
 * JSMESS (in-browser Javascript implementation of MESS, try it online)
 * Hercules (open source, System/370, ESRA/390, z/ARCH, website)
 * QEMU (open source, generic, website)
 * KEEP - Keeping Emulation Environments Portable (open source, generic, website)
 * Emulation Framework (SourceForge site)
 * TOTEM - Trustworthy Online Technical Environment Metadata Database
 * SIMH (open source, multi-system, website)
 * Emularity (open-source platform to embed emulators, website)