Open source code review system!

I have just wrote in my blog yesterday that I want to build an open source code review system on Google apps engine. And today I am already reading Guido van Rossum's Rietveld . It is based on the code review system "Mondrian" that's widely used in Google, which is also developed by Guido.
I've just tried it out with a patch I made for WebDriver. Worked pretty well even though the user interface looks still in early stage. I'm going to checkout the code to see more about it now.