There are many ways to get a cool YouTube Video on your hard drive. But sadly most websites or programs who offer such a service are packed with advertising or spyware. So I wrote a quick small program who downloads YouTube videos.
There is nothing special about it. The key thing is to obtain a valid token and pass it to a special url:
Token = html.Substring(html.IndexOf("\"t\":") + 6, 44);
DownloadUrl = Uri.UnescapeDataString("http://youtube.com/get_video?video_id=" + VideoId + "&t=" + Token + "&fmt=" + Format + "&asv=2");
You can get the source code here.