Three random use cases:
* Medical and pseudo-medical practices
* Oven cleaning companies and other property maintenance services
* Advice services
Does something like Google Calendar work well for that? You could use it when you have a diary secretary in the way, but not if you want a self-service element.
She primarily sells to hair stylists and those people, but has lots of different customers that use it in pretty out-of-the-box ways...
I know a company that build billing for medical and they tried to build out a product like this but the people running it just hated it. They either get people all booked in horrible times or for a non-busy place people see that and associate it with being bad. One solution they never tried is trying to make it like the Apple store booking engine.
genbook.com seems to have some decent traction in the space