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.
Jama Connect - The Leader in Requirements Management Solutions
KanbanMail - A Kanban board for your emails.
ReqView - Simple and powerful requirements management tool enabling easy requirements gathering, traceability tracking and offline collaboration.
Sunsama - Calendar and scheduling for teams
Serena Dimensions RM - Serena Dimensions RM is a web application to improve the definition and management of requirements across the product lifecycle.