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.
UserVoice - UserVoice integrates easy-to-use feedback, helpdesk, and knowledge base management tools in one platform that empowers users to speak and companies to understand.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
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
Slack - A messaging app for teams who see through the Earth!
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.