Hula Server - gmail-ish Calendar and Email server

Hula Server - Hula: "Hula is a calendar and mail server. We are focused on building a calendar and mail server that people love to use, instead of broadly trying to build a 'groupware server' that managers want to deploy.
Hula is available under the LGPL (http://www.gnu.org/copyleft/lesser.html) and MPL (http://www.mozilla.org/MPL/).

Calendaring
We want to build a real web-based calendar: to make it trivially easy to publish a calendar, to invite anyone with an email address to an an appointment and process their RSVPs, to get to your calendar via HTML or RSS or with an instant messenger or with SMS. We think it's time someone rethought calendars, and we have a lot of new ideas.

Rich Web Client
We want to build a high-quality JavaScript/DHTML web interface for both mail and calendaring, in the style of Google's GMail (http://www.gmail.com/). "