Sunbird, Mozilla's calendar application, supports extensions just as Firefox and Thunderbird do. What kind of extensions work with a calendar? How about being able to get a weather forecast when you're setting up a golf date, or exporting your desktop calendar to a Web service?
Sunbird is a calendar application built by the people at the Mozilla Foundation. It's entirely standalone: it doesn't require the bulk of another application, but doesn't offer any integration into other programs.