Include Widget plugin by karrde includes a file’s contents in the widget.
All this plugin does is use the PHP include function to include the specified file in the widget. There is NO checking done whatsoever on the file.
The author says “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.”
Download: http://wordpress.org/extend/plugins/include-widget/