Include Me
Include Me helps to include in posts or pages external files usually to be shared between different posts or pages or that contains PHP or other code that can be compromised by the visual editor.
The use is immediate: the shortcode [includeme] is all that you need (see the documentation on Include Me official page).
The best way to use it is to include functionalities written in external PHP that will be rendered in post body or to include pieces of javascript that will be hard to add with WordPress editor.
Inclusions can be rendered with IFRAME if needed to create boxes that display external web pages.
This plugin is made of few line of code, ultralite!
Other plugins by Stefano Lissa:
Translation
You can contribute to translate this plugin in your language on WordPress Translate