I like to use Gravity forms as an alternative to the WPMU plugin i'm currently using (http://premium.wpmudev.org/project/appointments-plus/).
Not all features should be possible of course, but what I would like to do is that my clients could choose from a predefined list of available time slots. Every time slot can be ordered only once. So once a time slot is booked (and confirmed - either manually or when paid), the option would not be available anymore for other users.
"Time slot" = for example: from 9 am to 10 am.
