Customer ability to book their own appointments

I run ServiceDesk for my part-time computer consulting business. I also use the same product every day in my full-time job as helpdesk Tech. In my day job since we support the company's internal and external employees, there is little need for scheduling beyond setting time off schedules and back up techs. Since we are on call and work at the office that we support there is really no need for my internal customers to require booking appointments.
In my business however, I go on-site, people drop off equipment, and I schedule support calls over the phone and remote desktop.
It would be wonderful for customers to be able to log into the portal and request their own appointment bookings based on the organization's operational hours and the appointments already scheduled for the techs. it seems like this functionality is mostly in place already since you can book vacation time and have backup techs handle your requests. Far too often I am not able to easily book an appointment when a client calls, and this would seem to solve many problems. I am very impressed with the booking mechanism that Apple has put online for their genius store - this is what got me thinking.
Customer logs in and sees available time slots, and requests an appointment and a tech can either confirm or suggest another time slot.
Chris Davis