My first project as a Google engineer was an internal web app for code review. According to Wikipedia, code review is "systematic examination (often as peer review) of computer source code intended to find and fix mistakes overlooked in the initial development phase, improving both the overall quality of software and the developers' skills." Not an exciting topic, perhaps, but the internal web app, which I code-named Mondrian after one of my favorite Dutch painters, was an overnight success among Google engineers (who evidently value software quality and skills development :-). I even gave a public presentation about it: you can watch the video on YouTube.
I've always hoped that we could release Mondrian as open source, but so far it hasn't happened: 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.
Fortunately, now that I work on the Google App Engine team, I've been able to write a new web app that incorporates many ideas (and even some code!) from Mondrian, and release it as open source. The Python open source community has been trying out Rietveld for the past few days, and has already been using it to do code reviews for Python (as well as providing valuable feedback in the form of bug reports and feature requests). Of course, the tool is not language-specific: you can use it for code reviews for any language!
To learn more about Rietveld, try it out, or take a look at the code, check out the article on Google Code!