Jira - The #1 software development tool used by agile teams. Jira Software is built for every member of your software team to plan, track, and release great software.
rmtoo - rmtoo is a free and open source requirements management tool.
Redmine - Flexible project management web application
Doorstop - Requirements management using version control.
Trac - Trac is a project management and bug/issue tracking system. Provides an interface to Subversion and an integrated wiki.
Requirements Quality Suite - The Systems Engineering Suite offers you the tools, processes and skills to Verification, Traceability, Reusability and Quality.