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
Reviewable
Code review tool for GitHub pull requests
Graphite
Code review platform for GitHub with stacked pull requests