The Optimizer team has been heads down, working towards the first release of Geocortex Optimizer for ArcGIS Server. It's been a great challenge. Like most software projects, there are never enough calendar days to implement the large number of good ideas we would like to incorporate into the product. The textbook I used in a recent project management course suggests that 1 day spent in planning is worth 4 days spent in implementation and testing. Given that I only have 20 business days to finish optimizer implementation, I computed I could stretch my implementation time to 65 days by planning for 15 and implementing for five. Just kidding, Steve.
I like to use UML class and sequence diagrams when designing features. To create them, I've tried both Microsoft Visual Studio and Visio. Visual Studio class diagrams look nice but unfortunately, Visual Studio has no support for sequence diagrams. Visio is ok, but really is more of a drawing tool than a design tool. The UML tool I settled on was Visual Paradigm for UML. It is a full featured, reasonably priced UML tool that despite a few quirks and a number of bugs that I've learned to work around, works quite well. If you're into UML, check it out. It might be what you're looking for.