JIRA

Isn't JIRA an issue tracker?

But

Can also be used for Requirements Management

Atlassian (creators) use Confluence + JIRA for requirements management

Atlassian's Workflow

• Use Confluence for internal discussion and general requirements gathering and project discussion.
• Create issues from requirements using Confluence's JIRA integration
• Enables viewing both requirements content and issues in line for the corresponding project.
• Confluence's wiki enables devs to edit requirements when necessary
• Keeps both stakeholders and devs current
  

Can also create a custom issue type for requirements

Allows you to use JIRA for RM w/o Confluence

With custom issue type, you can define different workflows than normal issues

Subtasks are a quick and easy way to add requirements and keep them linked together

Video Demo