Review on Software Ideas Modeler by David Everett

I love that it can generate diagrams from code easily without having to write any extra logic or boilerplate classes/structures for each type of object you want to model (elements in an app).

It's not free though - but they have great customer service who are able to help at very short notice if needed! You need some experience using this tool before deciding whether its worth paying up front as there isn't much documentation out-of-the box which makes learning how best use difficult once your project starts progressing. We're solving problems relating directly back into our business process by modelling what we do currently internally so mapping those processes onto new products being developed was key here.