16bugs - 16bugs is an all-in-one bug tracking software that makes it easier for you to manage all sorts of bugs in performance with the simple interface; ultimately, you will be more productive for sure.
Bugify - Bugify is a simple PHP issue tracker, designed to offer powerful bug tracking capabilities in an easy to use system.
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.
Traq - Traq is a PHP powered project manager and issue tracker featuring multiple projects, milestones...
Linear - Streamlined issue tracking for software teams
Jira - The #1 software development tool used by agile teams. Jira Software is built for every member of your software team to plan, track, and release great software.