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.

Roadmap - Collision avoidance for projects and people

ReqView - Simple and powerful requirements management tool enabling easy requirements gathering, traceability tracking and offline collaboration.

productboard - Beautiful and powerful product management.

Visure - Visure offers  a flexible , modern and complete requirements ALM Platform (Application Lifecycle Management).