Emulators

Fuse is a free and open source multiplatform emulator of Sinclair ZX Spectrum with support for all original models and some clones

melonDS is a free and open source multiplatform emulator for Nintendo DS and DSi that provides fast and accurate emulation

GB Enhanced+ is a free and open source a Game Boy emulator

Hatari is a free and open source multiplatform emulator for Atari ST/STE/TT/Falcon

DOSBox-X is a free and open source multiplatform DOS emulator for running DOS applications and games

DuckStation is an simulator/emulator of the Sony PlayStation(TM) console, focusing on playability, speed, and long-term maintainability

Dolphin Emulator is a free and open source multiplatform emulator for the two latest Nintendo game consoles: GameCube and Wii

DeSmuME is a free and open source multiplatform Nintendo DS emulator

bsnes is a free and open source multiplatform Super Nintendo emulator focused on performance, features and ease of use

Citra is a free and open source multiplatform experimental emulator/debugger for Nintendo 3DS