Hidariue

Hidari means left, Ue means up.
On the upper lefthand corner of NicoNicoDouga website, there is a small animegif icon which changes each time you load, which is called "Hidariue" icon.

For example, this is Hidariue icon.
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You can see the list here:

You can download some of them here:

Some people came up creating videos using this Hidariue icon like this one:

【ニコニコ動画の左上にナイト・オブ・ナイツを合わせてみた】

For the 4th MMD cup, this hillarious video is copying the Hidariue icons and making the MMD characters perform those motions. 

【第4回MMD杯本選】ニコニコ天国と地獄