rmtoo - rmtoo is a free and open source requirements management tool.
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.
Doorstop - Requirements management using version control.
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.
Requirements Quality Suite - The Systems Engineering Suite offers you the tools, processes and skills to Verification, Traceability, Reusability and Quality.