Internet is such a great place. I have been watching a lot of educational/self-learning videos on youtube.com. Many times there is a video I want to watch again but Internet access is not optimal at that time or I may have forgotten the url to that video. For this reason I download these videos and watch them at my convenient time.
Last week I tried a number of tools and finally settled on using download helper. Download helper is a Firefox extension which allows you to easily download any video from youtube.com and many other video sites.
A download helper icon appears next to the home page icon (see image below) with a drop down list of all videos on that web page. Select the correct video from the drop down list and it will starts downloading.