In Download YouTube Videos as MP4 Files post, I had written about how you can download YouTube video as an MP4 file to your computer.
Here is an updated bookmarklet for downloading YouTube videos.
To use it:
Right click on the above link and select “Bookmark this link” in Firefox or “Add to favorites” in Internet Explorer. (Select “Favorites Bar” in Internet Explorer or “Bookmarks Toolbar” in Firefox, and to make it easily accessible via the toolbar in these browsers).
When you click on the link, it should open up a dialog to save the mp4 to your computer. A good thing about this bookmarklet is that it detects and automatically downloads the HQ video if it is available.
PS: Remember to use it according to Google’s user agreement policy.