Google has launched a new code review feature called "assigned reviews" for improve project hosting. Now projects that use code reviews have two choices: post-commit reviews for code reviews after submission, and assigned code reviews for all code heading into trunk. Of course, you can also not use any reviews at all. Use whatever style or styles that work best for your team.
How does it work? Commit your code to a branch of your choosing, then create a new "Type-Review" Issue requesting that another team member do a code review for your branch. Review requests can be labeled, commented on, and assigned (or reassigned) like any other issue. Once created, review requests show up both in your project's issue tracker, and in a new table at the top of your project's recent source code changes list.
More info: Code Review Documentation