Lightning brings the Sunbird calendar to the popular email client,
Mozilla Thunderbird.

Mozilla Thunderbird is a redesign of the Mozilla mail component with the goal
of becoming a cross-platform stand alone mail application using the XUL
user interface language.  See the Mozilla Thunderbird project page for
more details.

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/lightning/
     http://www.mozilla.com/thunderbird/
     http://www.mozilla.org/projects/calendar/sunbird/