Review Assistant - Review Assistant is a code review plugin for Visual Studio.
Gerrit Code Review - OpenSource Git Code Review Tool
MergeBoard - MergeBoard is the first code review tool to display code changes in a smart way. Its unique features help developers spend less time on code reviews and catch more bugs.
Phabricator - Phacility - Phabricator
Upsource - Upsource provides unified access to all your projects stored in Git, Perforce, Mercurial or Subversion.
GitHub - Originally founded as a project to simplify sharing code, GitHub has grown into an application used by over a million people to store over two million code repositories, making GitHub the largest code host in the world.