How to download video from youtube.com and any similiar website?

How to download video from youtube.com and any similiar website (Google, iFilm, etc)? Even the website is not allow us to download or save their video..hehe.
I found this way by googling. Open the website, click the video you want to save, and see the link in your web browser, copy the link and here’s the monster, hehe : open www.keepvid.com (thanks so much to you), and simply paste the link into the bar that asking the link, and then choose the web where the video comes from, click the download button in the right, wait a sec…the link for downloading is soon appear below. Simply click the download link, and the video is saving to your computer.
Have a nice video hunting! ^^