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?
World Weather+ is a great extension that grabs weather data from the Weather Channel. It displays the current conditions on the left side of Sunbird, below the task section. To get started, download and install the add-on, select the add-on's preferences, and provide it your ZIP code. It will then display your location's current weather conditions along with the option to view the forecast for the next nine days.