You could use http://keepvid.com/ but there are many other: http://www.google.com/#hl=en&q=save+youtube+video
You can download any grabber which are available on net for free. My best one is OrbitDownloder or Ant
go to download youtube videos and paste your link to url download. Select the size you want that is all
Hello, there are many ways to download YouTube video, and here is a nice guide to do that - http://flash-video-player.blogspot.com/2010/08/download-youtube-videos-to-your.html
What i personally like to do is not to use any software, but just copy and paste the flash file from the temp cache folder of windows/linux/mac osx, rename and then save the file For windows follow this walkernews.net/2007/11/19/how-to-save-flash-video-from-firefox-cache-folder/ For ubuntu go to /tmp/randomfile that random file name will show the thumbnail of the video you want to save once its finished download fully, then you can grab that file and rename it to your need. Good luck.
You can see my blog post, a youtube download script for firefox, chrom & opera http://tesarn.blogspot.com/2010/08/youtube-video-download-script-for.html