For most people, the first thing they think of when it comes to online video is Youtube.  Youtube is great.  It's cool.  It has ridiculous amounts of stuff.

It's not Nico Nico Douga.

Most people know about the scrolling comments, which can add greatly to the enjoyment.  But did you know about the looping?  Pretty damn cool.  Here's a pretty innocuous example, while this one is a considerably less innocuous example.

Okay, so maybe you did see a couple of these before, but there's more to Nico Video than comments and looping.  Have you seen this video?  Try pausing it.  I laughed way too hard at this.

(I actually spent a considerable amount of time trying to find a non-anime MAD example of that pause functionality.  No go.  Why does that make me sad?)