CheckUser

CheckUser is an extension that lets wiki administrators check user accounts to see if they are using the same IP address.

This extension useful to determine if one or more user accounts are being used by the same person for the purposes of vandalism, spam, trolling, sockpuppeting, or other potentially undesirable activity on the wiki.

It's use requires a special "checkuser" right (which can be assigned to trusted parties), and unless the wiki is a localhost wiki with one user and/or a private "by invitation only" wiki, this extension is pretty much invaluable to any wiki administrator of any skill level.