Mozilla Sunbird™ is a standalone calendar application. Sunbird is being developed for some years. The latest version is 0.7 (far away from the milestone 1.0 which means that some bugs should probably be expected) which you can download here. Sunbird has small system requirments and runs smoothly without consuming much memory.
The interface is simple but usefull. You can view your calendar in day, week, multiweek or month mode. Toolbar is customisable as well (like in Firefox). Adding events is easy: Just click the “New Event” button on the upper left corner, fill in the details (duration, start time, end time), choose wether or not you want to be notified (Sunbird should be running for notifications to work) and click Save and Close. There is an option for repeating events too.
Next to the “New Event” button there is the “New Task” button. Pop-up window for task adding is the same with the event add, but after saving, tasks are listed in the lower left corner, giving you the ability to give them priority, fill the fullfilment percentage and tick the fullfiled ones.
For social people, Sunbird gives the option for exporting your calendar (or importing a calendar) and publishing your Calendar so it can be shared with others. Calendars can be password protected for more security.
In general Mozilla Sunbird™ offers the basics of a Calendar application and is suitable for the home user or users that do not need advanced features from their calendar app. I’ve been using Sunbird 0.7 for two weeks now and it is really stable (has never crashed) which should be considered to pros.