rmtoo - rmtoo is a free and open source requirements management tool.
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.
Cradle - 3SL Cradle is a requirements management and systems engineering software tool.
Jama Connect - The Leader in Requirements Management Solutions