I've tried a number of them and Code Collaborator is an excellent stand alone code review tool. It imports well from many source control systems, but didn't integrate seamlessly with my bug/feature tracking system. They have a new version out since I used it, so perhaps that has been improved. Code Collaborator has a beautiful comment / issue / suggestion tracking system.
Fogbugz is a great lightweight feature/bug/customer tracking system for software development. It also now integrates really well with Kiln, a Mercurial-based source control and they've recently added a pretty cool code review tool.
What I like best about the Kiln/Fogbugz code review process is that it works post-checkin and it enables the submitter to request a code review from a peer and automatically create a work item assigned to their peer to track the code review. This is imho an excellent combination of all the code review best practices rolled into one.
With Code Collaborator, I struggled with the 'assign a review to' logic and because it works against diffs/changesets rather than checked in code it tended to impede lines of code written with a bit too much process. You want to ensure your code review process is a 'teaching tool' rather than a 'quality gate' otherwise the 'gate' slows things down and you start to lose acceptance from your devs.