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.
3dmoo - 3DS Emulator Prototype
Drastic - Drastic is a Android DS Emulator
No$GBA - If you have no money you can play Nintendo DS and Gameboy Advance game.