Beanstalk - private repo hosting for business - just released a brand new code review tool that we've been beta testing for a while now. It's rather nice and addresses many of the rough edges of pull requests. It's definitely for same-company teams as opposed to large open-source collaborative projects, though.

http://beanstalkapp.com

We host all of our private repos with Beanstalk, and use GH for our open source repos.