IBM (formerly Rational) RequisitePro is a powerful tool for writing software requirements.
This tool is most effective when you are managing hundreds of requirements. Authors write with MS Word, and the requirements become part of a database which provides flexibility in retrieval and reporting.
stpBA Storyboarding integrates requirements management with diagramming of task flows and specifications. It can be used as a standalone tool, or integrated with IBM RequisitePro.
Omnigroup has a suite of tools for the Macintosh. OmniOutliner is the application for writing requirements, and it integrates with OmniGraffle, a popular visualization tool.
The Confluence Wiki is being used for collaborative work, including writing requirements.