SelectCategoryTagCloud is a Mediawiki extension. The URL is https://www.mediawiki.org/wiki/Extension:SelectCategoryTagCloud

The SelectCategoryTagCloud extension provides five functions:
# It shows a tag cloud of all categories (unless limited by minimum count or by a custom root category)
# It strips all categories linked within a page upon editing and inserts them in a category input box.
# It adds categories selected from the tag cloud to the input box and removes them if already there.
# It suggests existing categories that match the user input in the input box from the database via AJAX
# It saves the categories from the input box to the text body of the page on saving.

For an overview of the extension also see the author's blog post on Open Spaces.

To see the extension in action, see MIKE2.0.