Kronolith is the Horde calendar application. It provides a stable and featureful individual calendar system with integrated collaboration and scheduling features. It makes extensive use of the Horde Framework to provide integration with other applications. It implements a solid, stand-alone calendar system, allowing repeating events, all-day events, custom fields, keywords, shared calendars, iCalendar support, generation of free/busy information, and managing multiple users through Horde Authentication.
GroupOffice is a groupware suite that takes your office online, implementing online collaboration and CRM. It allows you to share projects, calendars, files and email online with co-workers and clients. It is easy to use and fully customizable. It also features synchronization with PDAs and Outlook.