Wantyoutube is a good way to download youtube videos! It supports video-sharing websites like: 5min, Break, Current, Dailymotion, Facebook,Google Video, ku6, Megavideo, Metacafe, Myspace, Photobucket, Sina, Spike, Tudou, Veoh, Vimeo, Yahoo Videos, fc2, Yam, Youku, Youtube, Xuite, Xvideos, 56.com. And many other sites is available, you have to try by your own!
In my view, the easiest way to download youtube video is to add the word "want" in video link. Like "http://www.youtube.com/watch?v=iCtZ8EMVJ60", you just change the URL to "http://www.wantyoutube.com/watch?v=iCtZ8EMVJ60"
This method is easy, free and "Wantyoutube" can download Mp4, 3Gp, which is available for your ipod, ipad, iphone, and other apps. If you just need MP3 (sound file only), "Wantyoutube" can do, too. It's a real useful tool for youtube videos.
I have tried it myself, and it really works!! Good luck!!