Zendesk - Zendesk is a beautiful, lightweight help-desk solution.

Azendoo - Azendoo is a collaborative task management solution that helps companies to get work done easily and in teams.

Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community

Taskade - Make lists, organize your thoughts, and be inspired to get things done. Taskade is a collaborative space for your tasks.

HelpScout - Help Scout is a simple, straightforward way to provide excellent support

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.