I guess most of us use youtube very regularly. If you find a good video and want to download and view it offline. There are lot softwares available one that works very well is
The tool is very simple to use.
Download & Install the software, If needed restart the browser.
Goto a particular youtube video which you want to download.
Select the Format you want.
Download and watch it offline.
You can also use VLC player to watch the videos offline.
VLC supports all formats.