Internet is like a Big Ocean. We can get all free software applications here.
Here I am giving one of the Firefox add-on to do download Video from famous sites like Youtube,Google Video, Myspace, Metacafe, Dailymotion, Veho, Blip.tv, Megavideo.
The add-on name is NetVideoHunter. Even You can download MP3 from online streaming sites.
You can handle the download application very easily. After installing it, you can see the tiny blue icon on the right hand side bottom of your Firefox browser.
For More Info and downloading : http://netvideohunter.com/