Ask HN: How do you capture domain knowledge from your users?

As a developer, when working for a client or on your own app, what tools/techniques do you use to capture domain knowledge from your users? For example when using Domain Driven Design, do you use Trello or another board tool to capture commands, queries, events etc...?

Most importantly (this is HN after all), what tool would like to have in hand to better capture this knowledge, and how automated should it be? Would you be interested in a tool that captures user needs and builds specs or even an API template, that users could try quickly for rapid feedback?