I'm a big fan of code reviews, so when another person expressed willingness to join the development, I immediately decided I want the technical means for doing the code reviews.
First, I tried the review engine at github.com. That didn't go well. It was easy to set up, but the review engine is laughable (sorry guys. Github is a great service, but code reviews are not, at this point, its strong side).
Then I installed reviewboard. It was not hard, and it's a huge improvement over github. However, it's still somewhat kinku to use, wants a local repository for Git, and I grew to strongly prefer online comments to small bubbles at the beginnings of the lines.
So, I gave a try to Rietveld, an open source incarnation of Google's Mondrian. It can be installed on AppEngine, and it was also ported to pure Django. Rietveld is immediately usable with Git, even without configuring any repositories, and it is as simple to use as it gets. And I got it working after maybe one hour of wrestling.
But now for the best part: it's actually available in Google Apps for Domains! To set it up for your domain, go to the administration dashboard, click the "Add more services" link and select the Code Reviewing engine. One CNAME record later, you're done. All the joy of Mondrian combined with zero administration and 3-minute setup.
Funny how the most powerful thing out there is also the easiest to use. Oh, the wonders of software as a service!