Source: http://lifehacker.com/5493662/get-youtube-bookmarklet-grabs-youtube-videos-in-one-click
Over at the technology site How-To Geek they've put together Get YouTube, a bookmarklet that enhances your YouTube experience by placing a simple download link in the sidebar of YouTube. It doesn't have any bells or whistles, you won't find any options for selecting formats or such, it just lets you download the video you are currently watching at the quality and size you're watching it at.
You don't need to install any add-ons, install Greasemonkey, or visit any specialty sites. Visit How-To Geek at the link below, drag the bookmarklet to your bookmark toolbar, and grab any YouTube video that catches your fancy. Have a favorite tool for capturing streaming media? Let's hear about it in the comments.