Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
In contrast to web browsers, e-mail and calendar applications have remained largely platform-centric. If you use Windows, chances are you also run Outlook. If you're on a Mac, you're probably using Mail.app or Entourage. Unix? PINE (just kidding). Although Mozilla Thunderbird is a strong, cross-platform e-mail client, many users have hesitated to drop their client of choice due to its lack of support for calendaring, agendas, and to-do lists. Enter Lightning.
Lightning 0.1 is available for Windows, Mac OS X, and Linux as an extension for Thunderbird. Currently limited in its functionality and not yet ready for a production environment, it does offer a handful of calendaring functions, all of which are integrated into Thunderbird.
Installing it is simple. Download the .xpi file and then install it into Thunderbird as you would any other extension. After restarting the e-mail application, you are rewarded with a changed interface.