A few extensions, hacks and a little elbow grease can turn your vanilla installation of Thunderbird into a powerful productivity app. Mark Shuttleworth recently professed to us that he uses Thunderbird for email and Evolution for calendar management. While Mark's confession was enough to push me away from Gmail's web interface and onto Thunderbird full-time, having separate apps for email and calendar management makes no sense. The Ubuntu Forums has a detailed thread that shows how you can get the most out of Thunderbird by using the Lightning extension and Google Calendar.
AU - Aussie readers will have already seen our reader submitted tip on using lighting to view calendar events in Thunderbird.