Code reviews are the rule at Google -- peer review reduces bugs, increases code quality, reduces maintenance cost, opens up team communication, and helps get the job done right the first time.
Like many open source developers, Google engineers used to rely on mail and textual diffs when doing code reviews. That made code reviews a drag. Mondrian, a web based code review tool, made the process much more efficient by presenting the diffs and comments right in our browser. Mondrian inspired the open source project Review Board, and led to Rietveld, and now the new code review tools are available on Google Code's Project Hosting.
Reviewing code in your project is simple: browse any source file or diff, double click on a source line to add comments, then publish your comments along with a general comment and score for the revision.
You can see code reviews in action on the code.google.com support project. So why are you still reading this? Learn how to use code reviews and don't forget to let us know what you think.