Malevich is a code review system that I wrote initially for our team because I was missing Monrian - a Google tool that I learned to love when I was there.
We've been using it quite successfully for a month and a half now. Here are a few stats, as of right now, just from my team (a few other teams at Microsoft started using it as well):
Change lists reviewed: 217
Review iterations: 706
Files reviewed: 2386
Comments made: 2968
You can read more about the project here: http://1-800-magic.blogspot.com/2009/01/malevich-introduction.html, or get the code and documentation here: http://www.codeplex.com/Malevich.