At Google, all production code must be peer reviewed before it is allowed to be submitted to their source control system. Since I'm no longer a Googler, I don't have access to their awesome infrastructure so I had to roll my own. Review Board is a pretty good code review system, and unlike the open source implementation of Google's internal one, you can host Review Board on your own hardware.
We use Subversion as our SCM. Out of the box, Subversion doesn't force code to be reviewed before submission, and I couldn't find any documentation about forcing code reviews through pre-submit hooks. Subversion's hooks are pretty powerful, and Review Board has a decent API, so I wrote my own. Hopefully this is useful to someone out there. It's written in PHP5. Put this in your subversion hooks directory, call it pre-submit, and make it executable.