Kronolith is the Horde calendar application. It provides a stable and featureful individual calendar system for every Horde user, with integrated collaboration/scheduling features. It makes extensive use of the Horde Framework to provide integration with other applications.
Right now, Kronolith 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. The calendar API that Kronolith uses is abstracted such that it could work with any backend, but right now we provide SQL (abstracted to support most databases, including MySQL, PostgreSQL, Oracle, and MSSQL via PEAR DB), and Kolab backend libraries.
If you are interested in helping develop Kronolith, or just want to ask questions and keep an eye on its progress, be sure to join our mailing list!