rmtoo - rmtoo is a free and open source requirements management tool.
Requirements Quality Suite - The Systems Engineering Suite offers you the tools, processes and skills to Verification, Traceability, Reusability and Quality.
Doorstop - Requirements management using version control.
Innoslate - Innoslate is a complete product lifecycle management tool integrated with systems engineering, program management, and product design capabilities.
Reqflow - Reqflow is a free and open-source tool for traceability of requirements across documents, designed to analyse documents with speed and efficiency.
QVscribe - QVscribe is a lightweight tool that allows engineers to quickly analyze requirements documents for vulnerabilities. Currently available for Microsoft Word.