Ya u can Download just our friend said " Using Internet Download Manager " otherwise google has been released the latest software which is named as "YOUTUBE HD VIDEO DOWNLOADER "
When u download and install this software.., it will be opened as a browser,, and inside youtube website.....
The most common advantage of this software is you can download the best quality of videos... 250 pixel videos will be able to download atmost 450pixel or 720pixels............
Which means it will better than the usual quality of videos........
Generally IDM Downloads what the browser windows plays in screen... Some times in videos Sound will goes faster than videos or video will go faster than sound..... Here No problem........
I have downloaded India Vs Austarlia World Cup(Semi Final)
and India vs Srilanka (Final)..... at the end of download it looks like a live broadcast of video in Star Cricket Channel......