http://www.keepvid.com is one the popular websites which allows downloading and saving videos directly from Youtube, Google, Metacafe etc.
The website uses Java Applet for fetching and saving videos. Java Applets are also used by several Commercial, Financial and Online Game websites. For running Java Applets, your browser should have Java Plugins installed.
It has been found that, default Ubuntu installation does not provide Java Plugin support for browsers (Mozilla and Chromium). To install Java Plugin, open Ubuntu Software Center (Synaptic Package Manager) and search ‘java‘ and install a Java Plugin, as shown in image below:
and Restart your browser. It should work!!