rmtoo - rmtoo is a free and open source requirements management tool.
QVscribe - QVscribe is a lightweight tool that allows engineers to quickly analyze requirements documents for vulnerabilities. Currently available for Microsoft Word.
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.
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.