if u r using real player latest vesion den when u would browse youtube vidsos it will show a download option ..click it for the download......... there are also some softwares like Get Youtube for downloading  YT  videos... and you can also download it from sonewebsites without using any downloading software  like http://videos.nkut.com but to play such download you need a software that can run that file.

there is also a another way to catch that file ..... but   kahee app

  na hoi