Doorstop - Requirements management using version control.
rmtoo - rmtoo is a free and open source requirements management tool.
Traceable Requirement Management - Product Managers and Requirement Engineers need to efficiently capture and communicate the "voice of the customer", to enable their organization to deliver the right offers at the right time.
Rational RequisitePro - The list below identifies the supported releases of IBM Rational RequisitePro from which you can select system requirements in different levels and organization of detail.
QVscribe - QVscribe is a lightweight tool that allows engineers to quickly analyze requirements documents for vulnerabilities. Currently available for Microsoft Word.
Reqflow - Reqflow is a free and open-source tool for traceability of requirements across documents, designed to analyse documents with speed and efficiency.