Scribunto

Scribunto (Latin for "they shall write" or "let them write") is an extension that allows for embedding scripting languages in MediaWiki.

Purpose
The extension allows for modules to be created in a namespace called Module. Currently, the only supported scripting language is Lua, which is known for its simplicity and usability.

Importance
Scribunto has embedded itself into the cores of the English Wikipedia so much that if the extension were suddenly disabled, thousands of modules would stop working and 70% of all pages (over 18,000,000) would suddenly have at least one thing broken. It is commonly deemed "one of, if not, the most useful MediaWiki extension ever".