Reviewable - GitHub code review platform.
Gerrit Code Review - OpenSource Git Code Review Tool
Phabricator - Phacility - Phabricator
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.
Review Board - Stress-free code review for teams of all sizes
BitBucket - Bitbucket is a free code hosting site for Mercurial and Git. Manage your development with a hosted wiki, issue tracker and source code.