If you like YouTube, you can see that you can't download any video from this site. Check this guide to see how easy is.
You need only the firefox.
Here are the instructions:
-First install Greasemonkey plugin for Firefox.
-Now go here , right click on script and select "install this user script". May you need to reload this page after installing the Greasemonkey script.
-No go to YouTube and select any video.In on top of the page, you will see the download button.