After having one of those 'Download Video from YouTube' sites many moons ago, i thought i'd check the code and see what could be done for Chrome as yes it can be done.
Create a new bookmark and call it "Download YouTube" or something you will remember, then for the address field paste:
Then save your bookmark. Next go to the YouTube video you want to download, when it starts playing click your "Download YouTube" bookmark and you will see the .flv start to download in Chrome's download status bar.
Source: Download YouTube Videos in Chrome - Google Chrome Forums
Create a new bookmark and call it "Download YouTube" or something you will remember, then for the address field paste:
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'];
Then save your bookmark. Next go to the YouTube video you want to download, when it starts playing click your "Download YouTube" bookmark and you will see the .flv start to download in Chrome's download status bar.
Source: Download YouTube Videos in Chrome - Google Chrome Forums