Some services involve a series of visits. Scheduling and keeping track of each visit can be a real pain. That's why it's important to incorporate subscription services in any scheduling system you consider.
Customer calls. You make their appointment for "sometime on Tuesday." Sound familiar? Believe it or not, they'd probably like something a little more specific than that...
Web-based appointment software is pretty common. There are literally dozens and dozens of service providers that will help you fill up your day with appointments.