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.
Serena Dimensions RM - Serena Dimensions RM is a web application to improve the definition and management of requirements across the product lifecycle.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
Visure - Visure offers a flexible , modern and complete requirements ALM Platform (Application Lifecycle Management).
Slack - A messaging app for teams who see through the Earth!
ReqView - Simple and powerful requirements management tool enabling easy requirements gathering, traceability tracking and offline collaboration.