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The best, non-scumware script is: http://rg3.github.io/youtube-dl/

I use "1-Click Youtube Video Downloader"[0] in Iceweasel on Debian unstable and it works great (it puts a "Download" button below the video between the Subscribe button and the number of views.

I've only used it a few times but it seems to work well. You can choose which video (resolution/quality) to download and then I usually watch 'em later using VLC.

https://addons.mozilla.org/en-US/firefox/addon/1-click-youtu...

Video-DownloadHelper[0] is also a good choice if you need to download videos often, and from different sources (e.g. blip.tv). However, it only works with Firefox. Jdownloader can also download Youtube Videos (and many others).

[0] https://addons.mozilla.org/de/firefox/addon/video-downloadhe...

I have used Keepvid.com before, it is simple and works good. Sometimes it requires the use of an Applet (which seems very suspicious to me) so I deny it access.

I use(d) http://userscripts.org/scripts/show/25105.