I need to provide some fairly complex functional specifications to my development team. I could use Visio, Word, or similar. However, I am also aware of products like LucidSpec that are built specifically for that task.
I've used a VS plug-in called GhostDoc which works well to add XML comments to provide some specs on what the function is doing, which is good for technical, which could then be reduced down to create functional docs.
We have some custom plug-ins for the Enterprise Architect (EA) to extend its off-the-shelf document generation and specification features. We need those extensions to support our custom specification language that bridges some gaps in our MDD approach. However, those extensions are very specific and the standard EA features would be sufficient in most cases. EA can track requirements connect them to use cases and so forth. You can specify basic dialogs and do all the other standard UML stuff.
As an analyst, I've tried multiple approaches:
I found out that in order to bring all the people to the table (it being the SAME table is a critical concern), you've got to have a system that is accessible to the business people (who own the requirements) and also by the development team.
And so when you want to have really good requirements, clarified beyond any doubt and agreed upon by everybody, it pays to have a system that the business people will use. No experience with LucidSpec; concerning GUI mockups, I have found from my own recent experience that non-imposing, playful apps such as Balsamiq can "draw" the business people into the game and get them think about what they want - great for gathering and analyzing requirements.
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[SELF-PROMO] You could test-drive Playground, a web app for writing requirements and specs that my company is operating. It's on the casual side and encourages cooperation between the owner / analyst and the development team.