Services such as Google Code have offered code reviewing tools as a part of their service. Their code review tool is so great that even Wikia uses it. Nothing like that has ever been available for MediaWiki...until now.
CodeReview is a MediaWiki extension by Brion Vibber, Aaron Schulz, Alexandre Emsenhuber and Chad Horohoe that provides a code review tool for MediaWikis. You can see a live demo on MediaWiki.org. Privileged users can "tag" commits as "ok", "fixme" and so on and (by default) everyone can comment on commits. Sure, it isn't as fancy as Google Code's (yet anyway ;-), but it's nice to have a "native" solution rather than having to use Google's or some other provider's services. Currently CodeReview extension seems to be requiring at least MediaWiki 1.13 or 1.14alpha and didn't work for me under 1.13alpha.
If you find yourself emailing your fellow co-committers more than often and you just happen to operate a MediaWiki instance, maybe you'd want to check out this extension. Granted, CodeReview isn't perfect and there still are a couple things that the developers would like to see fixed. But trust me, it's quite good start for a code review tool for MediaWiki. I'd most certainly recommend trying it out if you have some moments, a MediaWiki and a SVN to spare.