You should look at inteGREAT Requirements Studio tool, full traceability from concept, through requirments, through use cases, and test cases, front to back. This includes reverse engineering missions. An exciting project was just piloted between  edev technologies and the Provincial government to ressurect a legacy application, back from the depths, back to the surface with all the trace-ability, attributes, business rules, test cases re-created! The traceablity is driven from changes and additions to the knowlege base, and attruibutes and reports are rendered from the knowldege base.

Blueprint (www.blueprintsys.com) has a wonderful tool called Blueprint Requirements Center. You can write textual requirements, build use cases, create use case scenarios and model them through a simulation. You can then use the use cases to build test scenarios. Additionally, documents can be stored on the server and be accessed through Blueprint Requirements Center. As good as Blueprint is, it's even better - others who are not Blueprint users can view and comment on your requirements through a tool called Blueprint Reader - it works very similar to Adobe's Acrobat. I am not employed by Blueprint but I do know some companies who are using it and have increased their productivity by using the software. It might be worth a look.