RetroArch - RetroArch is a frontend for emulators, game engines and media players.
Citra - Citra is a work-in-progress emulator for the Nintendo 3DS.
melonDS - A Nintendo DS emulator with support for commercial games.
TronDS - A Nintendo 3DS Emulator, It's very simple and is only capable of running homebrews.
Drastic - Drastic is a Android DS Emulator
CD-i Emulator - The CD-i emulator program (cdiemu) provides a fairly complete emulation of the hardware of an actual physical CD-i player.