The problem with a public scratch pad is that it is public, and anyone can just come along and wipe it. If that's ok with you, feel free to use Whats a wiki?. Less interesting is the publicly readable but writable only by gods Cloud nine.

<injoke> Of course, we all know that if there were such a thing that were accessible only to, say, acolytes and above and writable only by abbots and above, it would be a productive place where much collaborative work could be done. </injoke>