SeaMonkey 2.0.
Lightning brings the Sunbird calendar to the popular email client, Mozilla
Thunderbird and the SeaMonkey internet application suite. Since it's an
extension, Lightning is tightly integrated with Thunderbird or SeaMonkey,
allowing it to easily perform email-related calendaring tasks.
WWW: http://www.mozilla.org/projects/calendar/lightning/
Thanks to: Andreas Tobler <andreast-list AT fgznet.ch>
The Sunbird Project is a redesign of the Mozilla Calendar component. The goal
is to produce a cross platform standalone calendar application based on
Mozilla's XUL user interface language.
WWW: http://www.mozilla.org/projects/calendar/sunbird.html
Note: This is still in the beta stages and you will probably run in to
a few bugs.