All Thunderbird add-ons that I consider essential for my work
Organize your schedule and life's important events in a calendar that's fully integrated with your Thunderbird email. Manage multiple calendars, create your daily to do list, invite friends to events, and subscribe to public calendars.
Use this extension to save things you see on the web into your Evernote account. It provides a toolbar button and context menu to add an article, selection, URL or an entire page.
Allows bidirectional access to Google Calendar
LookOut is a plugin which allows Thunderbird to interface with Microsoft's mail tools by decoding metadata and attachments encapsulated/embedded in a TNEF encoded attachment (aka winmail.dat).
Quickly get to the latest nightly build of Lightning relevant to the version of Thunderbird and the OS you're on.