The Web Appointment Scheduling System (WASS) is a system for scheduling and managing appointments over the Web. WASS includes a comprehensive system for creating/displaying appointment schedules, and an email and iCal appointment notification facility.
I haven't looked at this in a while, so use at your own risk.
Appointment Keeper is a web-based appointment scheduling system intended for small medical offices. It allows for multiple providers and a variety of searching and sorting functions.
Narra Appointment Scheduler is an online web-based, multipurpose, multilingual appointment scheduler. The project itself is purely javascript and will work independently with any server-side scripting language and database.