Well i recently downloaded the firefox plugin "Video downloader". it allowes you to download videos from video search engines like Youtube, Google-video etc
here the link to its webpage:

http://javimoya.com/blog/youtube_en.php

It really works great, especially if you like to view a video from your desktop.

All you need then is a video player that alowes you to watch the downloaded file ( its mostly a .flv file )

I then can recommend nFLV player :

http://www.download.com/nFLVPlayer/3...ml?tag=lst-0-1

Its all free and very handy :D