Connect Daily is a Web calendar and resource management application. Features include unlimited users, email reminders, public and private calendars, AJAX support, RSS, CSV import/export, and iCal support. Planner views and approvals prevent double-booking resources and make managing your facility a snap. A powerful security system provides flexible permission control. Authentication via LDAP or Active Directory is supported.
E*Reminders is a series of PHP scripts that interact with a MySQL database to setup email reminders for any events or occasions you'd like. It includes recurring reminders, advance notices, and password authentication for email addresses. It can be used for an entire company, as a service for Internet users on your site, or privately.
LowCal is a lightweight Web-based calendar of events. It is designed to allow you to announce events that have already been planned (it's not "groupware" and doesn't do "calendaring") without requiring the installation of PHP or a database backend. Events can be categorized and users can choose to display one, multiple, or all categories. Viewing modes include tabular monthly calendars and plain event lists, with or without details. Recurring events are well-supported using a cron-like syntax. All display is template-based, allowing complete configurability.
WebCal is a free browser-based calendar program to help you keep track of your appointments, meetings, birthdays, etc. It's simple, yet has powerful scheduling features, email reminders, and a user-friendly interface, and being Web-based makes it ideal for use by groups of people.