These are the best plugins for making Thunderbird most useful! Also check out Firefox Pro Pack.
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.
Collector's Note A calendar that can be used to invite attendees via email. Optionally configurable with various providers.
Allows bidirectional access to Google Calendar
Collector's Note Allows use of Google Calendar with Lightning.
Minimizes windows into the system tray.
Collector's Note Thunderbird stays in tray.
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).
True "Send Later" functionality to schedule the time for sending an email.
Synchronizes contacts & groups between Thunderbird or SeaMonkey and Google Contacts. Imports Facebook friends, Plaxo contacts, and more.
This extension allows you to manually sort (order) your folders in the folder pane of Thunderbird or automatically sort them, but in a better way. This extension also allows you to re-order accounts in the folder pane.
Add Dropbox support to Thunderbird's Filelink feature.
Collector's Note Use Dropbox with Thunderbird for sending large files.
Displays icon for user agent of received mails.
Collector's Note See what device sender was using in email.
Ever sent mail from the wrong email address because you forgot to select the correct identity in the compose window?
Collector's Note Simplify use of email aliases.