youtube-dl

Augmentable with front-ends such as https://alternativeto.net/software/youtubedlg/ , youtube-dl is a small command-line program to download videos from http://alternativeto.net/software/youtube/ and a hundreds other sites. It requires the Python interpreter and is therefore platform-independent. It can be integrated in other programs and so be used to watch online videos in local applications such as http://alternativeto.net/software/mpv/ .