KeepVid is what I personally use as the site is clean from any malware/spammy advertising and allows you to download Youtube videos in a number of different formats without downloading any application.
Opera, Firefox and Google Chrome offers plug-ins to download it without using any other third party application
Why are you all using applications to download youtube videos when there is a faster and easier means directly from youtube. First step: Click the video you want to download Second step: Goto your browser url where you see "http://www.youtube.com/hsuybuyebu" which is the videos url Third step: Delete the http://www. and replace it with ss , looking like this "ssyoutube.com/hsuybuyebu" the click enter Final step: You will be redirected to savefromnet where you will be asked to download the type of file whether .flv or .mp4 and so on. Just click the filetype and it will download automatically