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.

Trello for Slack - Organize anything with anyone, inside Slack

Roadmap - Collision avoidance for projects and people

Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.

productboard - Beautiful and powerful product management.

Halp - Slack-first ticketing for internal helpdesk