Bugify - Bugify is a simple PHP issue tracker, designed to offer powerful bug tracking capabilities in an easy to use system.
GitHub Reader - A quick way to browse GitHub issues and pull requests.
jtrac - JTrac is an open source and highly customizable issue-tracking web-application written in Java.
Shrink for Github - A macOS app for your Github issues
PlayNice.ly - PlayNice.ly is a blazing issue tracking software, making it easy for you developers to find out all the major bugs and errors.
Codetree - You can manage your entire development process through GitHub.