Starting in 2014 I built Reviewable (https://reviewable.io) because after getting used to Critique at Google I couldn't stand the workflow of GitHub's PR code reviews. Reviewable gives you interdiffs (even when rebasing), tracks what you've already reviewed, has a rich comment resolution system for multi-party reviews, and lets you know when the ball is in your court. It has also grown more customizable over the years, including plugging in snippets of custom code to, e.g., group files, determine if a file is reviewed, or determine if a review is complete.
It's not open source because, frankly, it's still hard to run a business on open source -- especially without VC backing (Reviewable is bootstrapped). After all these years things are still growing nicely and while I've run the show solo for a long time I started hiring full time folks in the last year or so. (If you think you might be interested to join, ping me and let me know!)