UnPlug is an extension that allows you to download video from sites such as YouTube, MetaCafe, and Vimeo. At your option, a button is placed on the right of the built-in search box in the browser, and when clicked, it searches the current page for media. Several options are presented, and you can easily download a viewable video to your computer. A flash player is NOT required for this plugin to work.
This even searches for video embedded with HTML5, including Theora, if it is available.