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Try https://reviewable.io then. Just code review, runs on top of GitHub, no extra repos to manage. (Disclosure: I'm the founder.)

Try Phabricator (http://phabricator.org/), you can point it at a remote repository without hosting it.

I'm confused. It syncs (some of) your repos and any PRs in them, so what else do you need to do with it besides code review instead of with GitHub?