Kumikyoku Nico Nico Douga

 is a series of video medleys, and is also the title of one of the videos in the series. The original video was created by Shimo (しも), a user of the video sharing website Nico Nico Douga.
Overview.
Kumikyoku Nico Nico Douga was first uploaded at 5 June 2007 with the title 「Nico Nico Douga中毒の方へ贈る一曲」 and continued to be worked on. A second version was uploaded on 23 June. The number of views skyrocketed to over 1.8 million after little more than 2 months. From July to August in 2007, the video maintained first position in number of views, with over 3.3 million comments, and was in over 100,000 (at the height of its popularity. Its popularity was thanks in part to subsequent videos.
Reception.
5 days after the original video was uploaded, another user uploaded a version with his own singing. After that, many users began to upload spin-offs (such as the play version, dance version) of the original video. Some of the lyrics from the original songs were improvised or mondegreen were used instead. Users sometime compile multiple user versions of the medley to form a 'chorus' video. Most of these "auditory illusions" were already well-known/prevalent on Nico Nico Douga.
The videos attracted a lot of singers who would add their own vocals to karaoke versions of the songs, the number of such videos far exceeded that of earlier videos. 
It's often misunderstood that the title is not a "music selection" in a strict sense, only "Nico Nico Douga medley songs used in it".
Some other videos posted include ones where users played instruments (recorder, piano, violin, a variety of drums and even bamboo flutes and other musical instruments) or an entire Orchestra performing an instrumental version of the song.
The medleys were also translated and sung in English, Hebrew, French, Korean, Chinese, Thai, Malay and Tagalog. Two such well known versions are the ones by C_Chat and the National Central University.
2 August 2007, C_Chat, a user of the Taiwan BBS board PTT Bulletin Board System, used a week of time to compile a group of people singing Kumikyoku in a chorus and also uploaded it to Nico Nico Douga. This is the first video based on Kumikyoku Nico Nico Douga outside Japan, and also let Japanese users realise that Nico Nico Douga had a lot of overseas users.
Lists of songs used.
The following is a list of songs used in the videos of the Kumikyoku Nico Nico Douga series.
Nico Nico Douga Ryūseigun.
Nico Nico Douga Ryūseimusume.
Although not officially in this version of the medley, the melodies of the songs Montagues and Capulets (Dance of the Knights) and You are the Prince of Tennis from The Prince of Tennis musicals can still be heard.
Nanairo no Nico Nico Douga.
The last section (ED1 - ED28) is also called "HEROES".
Others.
On 26 December 2007, Lantis published 「らき☆すたRe-Mix002〜『ラキスタノキワミ、アッー』【してやんよ】〜」, a "mixed music collection" based on Lucky Star. One of the songs in the collection, 「組曲『らき☆すた動画』」, was to pay tribute to the creativity of Kumikyoku Nico Nico Douga.
5 March 2008， the same company published a similar music CD called “”.
『ランティス組曲 feat. Nico Nico Artists』も 「組曲『ニコニコ動画』」
Somewhere around 19 October 2008, a Super Mario World ROM hacker created an automatic level in the game that would play the song using sound effects from the game (with the original song playing in the background) The video has gotten more than 2 million views across all the reuploads of the video on YouTube, and has become an important part of the Super Mario World ROM Hacking community.
In 2012, a team of users on YouTube collaborated to create "8 Sides of Nico Nico Douga", a remix of Nanairo Nico Nico Douga using voice samples from Jack Black's appearance on "Sesame Street". The second part of "RAINBOW GIRL" was replaced with "Karate" by Tenacious D.