YouTube is a video hosting platform, considered the largest one in the world (see its Wikipedia article).
Most YouTube videos are not eligible for upload to Wikimedia Commons.
However, since June 2011, YouTube offers the option to license videos under the free license Creative Commons Attribution 3.0 (CC-BY) which is compatible with Commons. These videos may be transferred to Wikimedia Commons, with the usual caution that you should not upload work that appears to be plagiarized or is out of scope.
Some other videos don't use this license option, but include it:
- in the video description (e.g. Wikipedia: Username, eligible for Commons and already here), or,
- directly in the video (e.g. Blu - Muto, not eligible for Commons as CC BY-NC-ND).
Search videos licensed as Creative Commons on YouTube by visiting this YouTube link and replacing "SearchText" with a different search term.
In May 2010, Google introduced the free WebM video encoding format, and many videos from YouTube are available under this format. As of November 2012, Wikimedia Commons accepts WebM uploads.
Moving YouTube files to Commons 
- Download and install python 2.7
- Download youtube2mediawiki (e.g. as ZIP-file)
- Extract the archive and use the command line to start the python script.
Alternatively you can request McZusatz to upload YouTube videos. Leave a message on his talk page including all the YouTube video IDs.
Just use the youtube-dl tool or JDownloader; both are also available as package in many GNU/Linux distributions. Alternatively a number of Firefox extensions are available, including BYTubeD - Bulk YouTube video Downloader.
Both BYTubeD (a Firefox extension) and JDownloader allow downloading all videos from a specific account (useful for large numbers of videos) and specific formats. If videos are available in .webm format, they don't need to be converted to a different format before uploading them to Commons.
See how to cut a WebM video in few seconds.
Scripts to convert to Ogg Theora and upload to Commons are available:
See also 
Related bugs 
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