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https://web.archive.org/web/20170530011511/https://github.com/gantt/downloadyoutube

A cross-browser script that adds a button to download YouTube videos as MP4 and FLV files. This script downloads the videos directly from YouTube and it integrates with YouTube's interface.

It creates a button under the video, to the left of "Add to".

Indispensable!

Abandoned, check out youtube-dl