Reviewable

Reviewable is the code review tool of choice for developers serious about code quality.  Quickly diff any two revisions -- across commits, rebasing, even squashing -- to see what changed since you last looked at the review, and automatically eliding uninteresting deltas.  Track the state of the review per file revisions (great for multi-party reviews), get a clear list of the comments you still need to reply to, and track discussions across code revisions until they're resolved.  Send comments in coherent batches to avoid email overload.  Customize the UI to optimize your workflow.  Sets up effortlessly and integrates with GitHub, with low learning curve and no maintenance unlike most existing open-source solutions.