Hatsune Miku performing The Secret Garden in ARToolkit_MMD with english subtitles.
Because i got fed up with too many video reposts coming from NicoNicoDouga.
Summary
ARToolkit detects a marker shape with a webcam and then uses that marker as the origin point for a 3D model (think facial recognition). MikuMikuDance was then paired up with the opensource toolkit to let you watch mikumikudance characters dance as you control your own camera angles with the marker.
This combination is known as ARToolkit_MMD.
(via & more http://www.youtube.com/watch?v=yLaPU1buB8A&feature=youtube_gdata)