Citra is an experimental open-source Nintendo 3DS emulator/debugger written in C++. At this time, it only emulates a very small subset of 3DS hardware, and therefore is only useful for booting/debugging very simple homebrew demos. Recent development has, however, allowed it to boot commercial games at a slow framerate. Such games cannot be played, however, as they crash soon after starting.
Due to its preliminary state, it has not yet had a stable release.