The year-and-a-half old Pull Panda provides Pull Reminders, Pull Analytics and Pull Assigner to improvde the code-review process. Pull Reminders allow developers to notify developers that a collaborator needs their review. Pull Analytics can provide stats on everything from wait times to top contributors. And Pull Assigner helps automatically distribute code across teams.
All three of these Pull Panda tools are available as of today for free in the form of a single GitHub Marketplace application called Pull Panda.