Ask HN: How do you collboratively document data flows and processes?

I'm looking for a tool that will help to document a data flow through an existing software system. I don't just want a diagramming or whiteboard tool, I want something collaborative that can be used by developers, product managers, support and others to collaborate in documenting their particular view of how it works and ultimately arrive at the 'correct' answer. Ideally the result, once ratified, would become the living documentation for the future.

I've used a few tools in the past, but none is ideal. GraphViz is great for data-driven diagrams but not collaborative and not friendly. Google Draw is collaborative but unsystematic.

I'm sure plenty of HNers have approached this kind of task. What tools or processes have you used?