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.
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.