FLV Player is required to play downloaded youtube videos. You can download FLV Player at http://applian.com/flvplayer/

Here is a list of olie resources that you may use to download youtube videos
http://www.youtubecatcher.com/
http://www.downloadyoutubevideos.com/
http://www.techcrunch.com/get-youtube-movie/
http://keepvid.com/
http://www.savevideodownload.com/download.php
http://www.vidgrab.com/
http://vixy.net/
I like http://vixy.net/ the most as it allows download in other formats as well

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