When a video is opened in youtube or any other site, where is it saved on my HDD,or how can I save that video????

What browser are you using? ... I have both chrome and firefox, and I know that google has an extension, and mozilla has an add-on (same thing really), that allow you to download video from youtube (or other sites that have video). with google look at their webstore and with mozilla go to mozilla.com and click add-ons. At both they have a search bar, Just type in YOUTUBE DOWNLOAD and you'll get a few results. Chrome has YoutubeOptions (Which I am in love it, you can disable ads, autoplay on or off, hide comments, and matte the background.) Firefox has YoutubeDownloader. With both you have the option of downloading as .FLV in either 240,360,or 480 -OR- as a .MP4. Chrome I think has default set to home/name/downloads, and mozilla will ask where you want to send downloads (And then use that folder from the point on.)

Now I am not recommending you go out and download all the simpsons and sopranos ... But that music video you saw on MTV last week that you MUST have ... Go for it. It's up to you to know what your doing after getting it Good luck

Hey I had come across this article http://ubuntuforums.org/showthread.php?t=547848 that told me the videos are saved in to the /tmp folder as they are being loaded. I had used the feature a lot back in Lucid with firefox 3.6.xx but now with Natty and Firfox 4 there are no videos in the /tmp folder. Can any one help me with this????

If you simply search the Firefox addons for "youtube" you will find many addons that are capable of downloading videos from youtube.com.

Whatever you browser gets stored in the cache directory of your browser. For firefox, try about:cache as your url.
In case of chrome you can manually define a cache directory where all the data gets stored using --user-data-dir=directory_path

Its due to flash updating where it stores tmp files-

http://askubuntu.com/questions/37267...-firefox-4-why

As an aside, with newer 64bit flash versions the video is broken into chunks, so if the youtube video is too long you cant copy and save the file from /tmp, no matter where its stored.