Jama Connect - The Leader in Requirements Management Solutions
Sortd - Rated the #1 App for Gmail
ReqView - Simple and powerful requirements management tool enabling easy requirements gathering, traceability tracking and offline collaboration.
KanbanMail - A Kanban board for your emails.
Accompa - Cloud-based requirements management software tool.
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.