YouTube world's number one Video-sharing website undoubtedly, ranked #3 for traffic. There are lots of ways to download videos from YouTube most commonly is by using cache-stream, downloaded using Internet Download Manger or any similar download manager (e.g. Orbit Downloader, Flash Get). You can either using online web-services like Savevid, KeepVid, CatchVideo.net
But a problem with all these is that every-time you want to download a video you either have to open all video pages and then let your Download Manager download them, or you have to paste video URL individually.
