Is there a plugin which allows me to save additional, invisible content per post?

I like this idea from wprecipes.com. It is a very lightweight solution.

add_shortcode( 'note', 'sc_note' );

function sc_note( $atts, $content = null ) {
     if ( current_user_can( 'publish_posts' ) )
        return '<div class="note">'.$content.'</div>';
    return '';
}

It allows you to use the [note]...[/note] shortcode to enter private comments.