If you use Opera, there are a couple of easy ways to save Youtube videos:
1. Since the Opera cache isn't encrypted, you can just go to your cache folder (C:\Documents and Settings\Saumil Singh\Application Data\Opera\Opera\profile\cache4) and fetch the .flv from there.
Note that it will have an extension of .tmp and so you will have to do a bit of hunting (arrange by size, time, etc.) to find the required file. Once you find it, save it wherever you want and change the extension to .flv.
2. Just go to your Local Settings\Temp folder while a Youtube video is still loading and there it will be as a .tmp file. Wait for the video to download fully then save this .tmp wherever you want and change the extension to .flv.
Just make sure you don't close the Youtube tab before saving the file, otherwise Opera will delete the file.