rmtoo - rmtoo is a free and open source requirements management tool.
Jira - The #1 software development tool used by agile teams. Jira Software is built for every member of your software team to plan, track, and release great software.
Doorstop - Requirements management using version control.
Valispace - Valispace is a modern browser based tool for complex hardware development that connects data for engineers, efficiently manages requirements and much more.
Reqflow - Reqflow is a free and open-source tool for traceability of requirements across documents, designed to analyse documents with speed and efficiency.
Jama - Jama combines requirement management software with enterprise collaboration capabilities to solve product delivery problems.