There are a number of tools/systems available aimed at streamlining and enhancing the code review process, including:
CodeStriker
Review Board, code review system in use at VMWare
Code Collaborator, commercial product by SmartBear
Rietveld, based on Modrian, the code review system in use at Google
Crucible, commercial product by Atlassian
These systems all have varying feature sets, and differ in degrees of maturity and polish; the selection is a little bewildering for someone who is evaluating code review systems for the frist time.
Some of these tools have already been mentioned in other questions/answers on StackOverflow, but I would like to see a more comprehensive comparison of the more popular systems, especially with respect to:
integration with source control systems
integration with bug tracking systems
supported workflow (reviews pre/post commit, review or contiguous/non-contigous revision ranges, etc)
deployment/maintenance requirements