There is little to be said about Mozilla Sunbird, the stand-alone calendar application. It has yet to hit its 0.1 milestone, and continues to use the old XPFE toolkit (rather than the new toolkit sported by Firefox and Thunderbird).
Sunbird is usable, however, unless you want to change preferences. The front-end is nearly indistinguishable from the standard calendar application. It includes support for multiple calendars (including remote calendars). Sunbird is also supposedly themable, but I haven't seen many themes available for calendar or Sunbird, and haven't tried it out.
Sunbird and its parent, the Mozilla calendar application, are still alpha quality (don't let Calendar's version fool you). If you want to check it out or help develop it, the project site is <http://www.mozilla.org/projects/calendar/>.