rmtoo - rmtoo is a free and open source requirements management tool.
Doorstop - Requirements management using version control.
Reqflow - Reqflow is a free and open-source tool for traceability of requirements across documents, designed to analyse documents with speed and efficiency.
Innoslate - Innoslate is a complete product lifecycle management tool integrated with systems engineering, program management, and product design capabilities.
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.
Jama Connect - The Leader in Requirements Management Solutions