I’ve used the Lightning addon for Thunderbird as my calendar for a year or so. Today I was browsing Lifehacker and found out that you can use the Google Calendar from Thunderbird. Lightning supports iCal which is also offered from Google Calendar, so it’s possible to import calendars to Thunderbird. However, it’s read only. If you create/edit events in Thunderbird, they won’t show in Google Calendar.
Fortunately there is a way to get around this limitation. There’s a Thunderbird addon that will let you to handle the calendar in both ways.

I’m thinking that I’ll replace the current forum calendar with a Google Calendar. It’s a lot more user friendly and more feature rich.

Thunderbird
http://www.mozilla.com/thunderbird/

Lightning
http://www.mozilla.org/projects/calendar/lightning/

Provider for Google Calendar
https://addons.mozilla.org/en-US/thunderbird/addon/4631