Phabricator
Suite of web applications for code review and software development
Phorge
Community fork of Phabricator for code review and project collaboration
Upsource
Code review and repository browsing tool from JetBrains (discontinued)
Collaborator
Peer review tool for code, documents and models
P4 Code Review
Code review tool for Perforce P4, formerly Helix Swarm
Rietveld
Web-based code review tool running on Google App Engine