I downloaded free trial version from sothink.com... its video downloader. you got 30 days...
it is running in the background and evertime it notices video file , small window pops up and ask you if you want to downoad it... When you go to you tube...you get the video without their watermark...I love it and use it to fill content on my site...
check it out
http://www.sothinkmedia.com/web-video-downloader/index.htm