AJAX Web-based Calendars

• WebCalendar: Impressive list of features, development work on Sourceforge
• Chronos: GPL licensed and Perl
• Citadel: GPL, written in C, full groupware suite with some AJAX features
• Monket: mostly CC licensed, PHP, AJAX.
• Zimbra: Java, AJAX -- MPL-derivative license
• Hula: AJAX email and calendar -- GPL license
• WebiCal: A Web application to view and edit multiple iCalendars -- GPL license

Closed source, free:

• CalendarHub: AJAX, RSS, Web 2.0, SMS, Import & Export, multiple personal calendars, calendar sharing & publishing
• Kiko: SMS, Instant Messenger, PDA, offline and pure HTML versions, iCal, whatever Outlook uses, AJAX, Web 2.0, Tagging, RSS (These guys are also developing open-source Javascript ActiveRecord^?, a JS implementation of Rail's ActiveRecord^?.)
• Planzo: AJAX, RSS, Web 2.0, XML, SMS
• Scalix: AJAX, email and calendar

Closed source, non-free:

• CalendarScript: Simple to use, extensible web calendar. Perl, so it works on all server platforms.
• Joyent: E-mail, calendars, contacts, file sharing, and shared applications. "Simple, powerful, web-based software for small teams."
• Trumba: "Trumba's mission is to connect people and communities through the events they share."

Other / unknown:

• Spongecell
• 30 Boxes
• AirSet