Vector

Vector is an extension that is supposed to work with the MediaWiki skin of the same name, and it makes it to where the sidebar menu is collapsible, which is handy if you have a ton of links on the sidebar and don't feel like scrolling down the page for a minute straight to find something on the bottom.

It only works for the Vector skin (but can be easily adapted for any MediaWiki skin based on Vector), and is highly recommended if you want a better navigable Vector sidebar menu.