Replace Text
From MediaWiki Beginners Wiki
Replace Text is an extension that allows the user to, well, replace text.
For instance, lets say you have a wiki where everyone needs to spell with British spelling, but the American version of color is on 80 or so pages when it should be "colour".
Well, this extension lets you find and replace whatever text you want with the text of your choice.
This is a really easy to use extension which can mass edit improperly spelled words or links, and if you don't have the experience to set up a bot (an automated wiki account which does maintenance editing in the background), this is basically the next best thing.