If you used a Firefox extension, or an Internet Explorer hack to enable you to download .flv or .mp4 versions of YouTube videos to your computer, and are missing this feature in Google Chrome, have no fear!
Just make a bookmark from either or both of these links to your bookmarks bar in Google Chrome, and just click the bookmark whenever you are viewing a YouTube video you want to download!
Download FLV:
javascript:window.location.href = 'http://youtube.com/get_video?video_id=' + swfArgs['video_id']+ "&l=" + swfArgs['l'] + "&sk=" + swfArgs['sk'] + '&fmt_map' + swfArgs['fmt_map'] + '&t=' + swfArgs['t'];
Download MP4: (High Resolution)
javascript:window.location.href = 'http://youtube.com/get_video?video_id=' + swfArgs['video_id']+ "&fmt=18" + "&l=" + swfArgs['l'] + "&sk=" + swfArgs['sk'] + '&fmt_map' + swfArgs['fmt_map'] + '&t=' + swfArgs['t'];
Source:
http://unlockforus.blogspot.com/Cheers!