DownloadHelper is an addon for Firefox. Search it on the Mozilla Firefox addon site. When you have it installed you'll get a small icon at the bottom of your browser. Whenever you go to most pages that have video the icon activates letting you know there is content you can download. It works great at youtube, just wait for the video to load, right-click on the DownloadHelper icon, go to Media, and you'll have options to download the video in various formats.