The one I built had the following features:
- people could make suggestions for how to improve the flow - email notifications when a flow was changed - document uploads tied to each step - tracking whether each user had viewed new flow changes, etc.
I have gone into a sales meeting and sold thousands of dollars of consulting services just by drawing out how this tool works on a napkin or scrap of paper. The idea is so simple and powerful that it is extremely valuable for businesses that care about process improvement.
I strongly recommend against listening to the 1% of people who get mad that it doesn't do all sorts of complex branching, etc.
I no longer work with the company I built it for (I was a minor partner) but I believe they still use it extensively, though it's focused only on supporting business/management consulting services and not on UI design, etc. We would routinely get $15-20 per user/month for it.