Do you have a great method for collection of and collaboration on project requirements documentation (product requirements, use cases, business rules, etc.) ?

I work for a small ISV (with about 10 engineers, 3 qa folks, and 2 business analysts). With a limited budget, we cannot afford tools such as RequisitePro. Using Word and a version control system just doesn't cut it. We've tried out using wiki and CMS tools to enable cross-linking between requirements (wich works out great), but these have limitiations when it comes to easy editing of documents by the non-techies, and significant short comings with exporting polished looking documents (to be shared with clients). I'm curious what tools/approaches other development shops in the same situation use for requirements gathering and analysis?

Free/Open Source tools would be great, but budget-friendly commercial tools are also a possibility. What combinations of cheap/free tools and practices have worked out well for you?