Video Download is a free add-on for Mozilla Firefox and Internet Explorer browsers, which lets you download videos from on-line sites like Youtube, Google Video, Grinvi, and similar, with just one mouse click.
This add-on creates a link to directly download from the right click (contextual menu). You just click on it to save it in the place you want in your hard drive. That means you now have it to watch whenever you like.
The instructions to install this add-on to Mozilla Firefox and Internet Explorer you can find on the author's website. It is free, simple and very practical.