Guido van Rossum, Python creator and member of the App Engine team, gave a talk on Mondrian which is the tool he created to work with the code review process at Google.
The Mondrian tool itself was tied to internal technology, but he took the time to built an inspired version on App Engine. Part of the work was having it work with Subversion as the SCM. The name of the tool is Rietveld, and here is what Guido said on the Python mailing list to announce it:
I've always hoped that I could release Mondrian as open source, but it was not to be: due to its popularity inside Google, it became more and more tied to proprietary Google infrastructure like Bigtable, and it remained limited to Perforce, the commercial revision control system most used at Google.
What I'm announcing now is the next best thing: an code review tool for use with Subversion, inspired by Mondrian and (soon to be) released as open source. Some of the code is even directly derived from Mondrian. Most of the code is new though, written using Django and running on Google App Engine.
I'm inviting the Python developer community to try out the tool on the web for code reviews. I've added a few code reviews already, but I'm hoping that more developers will upload at least one patch for review and invite a reviewer to try it out.
Brady Forrest has a nice write-up of the announcement and Joe Heck pointed out that this is not the same as another open source solution called Review Board.
For more details on the tool check out Guido's article.