There are lots of method for download you tube videos.But here i am listing some commonly use methods
(1)It`s very easy , Download youtube downloader from here.& install it.open software & copy & paste video url into software.BUT here is problem, if your connection lost then it can not be resumed.
(2)Copy video url from youtube, now open http://keepvid.com or http://savevid.com and paste video url into their site. But for this purpose you should have java program installed.
(3)it`s my favourite method. No software or plugin needed. If you want to download following video
http://youtube.com/watch?v=Y4E9brXa6hA then just put ss at begining of url like http://ssyoutube.com/watch?v=Y4E9brXa6hA and copy it & paste it in your browser addressbar.
(4)if you have internet download manager installed.it will automatically download when you start watching video. you can download full crack version of internet download manager form here.