HabuL is a Thunderbird addon that reports spam to the SpamCop, the FTC, FDA, SEC, ACMA, and Knujon.com. It also allows you to put in your own custom addresses to report spam to such parties as your ISP or corporate abuse address.
The OXtender for Thunderbird is an addon for Thunderbird (and Lightning) to provide users full access to appointments and contacts stored in the Open-Xchange Server and use Thunderbird as a rich client online and offline.
Lightning is a calendar application which works with the popular Mozilla Thunderbird and SeaMonkey mail clients. It lets you manage multiple calendars, create your daily todo list, invite friends to events, and subscribe to public calendars.