Rietveld starts off with major geek cred because its primary author is Python's Guido van Rossum. The tool is written in Python (of course!) and hosted on Google's own experimental cloud-computing framework AppEngine. Anyone with a Google account can participate; after logging in you can identify Subversion repositories and branches containing code that needs to be reviewed, upload changed source code files, and invite reviewers. Reviewers can see an online diff and leave their own comments, and the system uses email notifications to keep the discussion moving along.
Rietveld Issue Screen

Rietveld is right now a bare-bones implementation, though it's already being used by a number of Python projects. If you main interest is in finding the best tool to use for code reviews, though, there are at least three other web-based open source code review tools that you ought to evaluate before committing to Rietveld.