Karrde

Includes a file’s contents in the widget. Quick and dirty, all this does is use the PHP include function to include the specified file in the widget. There is NO checking done whatsoever on the file, so use it at your own risk. I wrote this because I couldn’t find a widget to do this ? plugins exist for using include in posts and pages, but not in widgets.

Include Widget

Includes a file's contents in the widget.

oltdev

This plugin enables page or post authors to scrape content from mediawiki pages and appending it to their pages. 
It works by scraping content from mediawiki pages by going to a special mediawiki page url.
For example http://en.wikipedia.org/wiki/Wordpress?action=render
The content rendered on the final page or post will always be the newest content, however it is not searchable via regular WordPress search form.
Use case: 
This plugin is great for adding documentation to your site. 
Since wordpress.org is using mediawiki for the codex you could potentially display wordpress codex pages on your site without much effort, just enter the full url of the specific wordpress codex page.
Enjoy

Wiki Append

Append a mediawiki page at the end of the regular wordpress page.

Include Widget vs Wiki Append