There's a cool feature where you can upload projects to the Scratch website and then embed them, perhaps into a blog post, or your own website. In the sidebar on your project's page, scroll down and click "embed." Then copy the second code down and paste it into your website.
If you don't want to upload to the Scratch website first, perhaps this topic will help you out: http://scratch.mit.edu/forums/viewtopic.php?id=191.
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