Global Content Blocks plugin by benz1 allows you to create your own shortcodes to insert reusable code snippets or HTML including forms, opt-in boxes, iframes, Adsense code, etc, into pages and posts. You can insert them using the shortcodes or via a button in the TinyMCE visual editor toolbar.
It is ideal for inserting reusable objects into your content or to prevent the WordPress editor from stripping out your code or otherwise changing your formatting. The snippets are masked as images to allow easy manipulation and non-html tags contamination.
Download and info: http://wordpress.org/extend/plugins/global-content-blocks/