NicoNico Douga

"Niconico (ニコニコ Nikoniko, lit. "Smile"), formerly known as Nico Nico Douga (ニコニコ動画 Niko Niko Dōga, lit. "Smiley Smiley Video"), is a video sharing website in Japan managed by Niwango.

Users can upload, view and share video clips. Unlike other video sharing sites, however, comments are overlaid directly onto the video, synced to a specific playback time. This allows comments to respond directly to events occurring in the video, in sync with the viewer; creating a sense of a shared watching experience."

Source: Wikipedia