Buglife - Seriously awesome bug reporting for iOS apps

Nifty - Manage projects, work, and communications in one place.

Linear - Streamlined issue tracking for software teams

Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.

Sifter - Sifter is designed to be a simple bug and issue tracker for small teams and works especially great for teams with non-technical folks involved.

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.