Go to http://www.youtube.com/html5, click on Join the HTML5 Beta link and search for a video. You have to use a web browser that supports the html5 video tag and the youtube video format, which pretty much limits you to Safari 4 or Chrome 4 by the time this article is published (Update: Firefox 4 is now on the list).

Eric Meyer demonstrates an interesting flexibility when html5 is used instead of flash.