From MediaWiki Widgets
The Widgets extension allows the creation of raw HTML pages that can be embedded (similarly to templates) in normal wiki pages. You do this by creating pages in the Widget namespace. They avoid the security problems of raw HTML in editable wiki pages because the privilege to edit in the Widget namespace is managed. Many pre-written Widgets are available and collected on this site. The extension is currently maintained by Yaron Koren.
For information on how to install and setup this extension as well as on how to create new widgets please refer to the extensions documentation page on mediawiki.org.