productboard - Beautiful and powerful product management.
Doorstop - Requirements management using version control.
ProdPad - ProdPad helps your team gather ideas, surface the best ones and turn them into product specs, and then put it all on a product roadmap.
rmtoo - rmtoo is a free and open source requirements management tool.
ProductPlan - Cloud-based product roadmap software solution
Reqflow - Reqflow is a free and open-source tool for traceability of requirements across documents, designed to analyse documents with speed and efficiency.