I use quite a few, actually. Here they are:

Minimize to Tray (for outlook express / the bat parity)
AboutConfig (adds a button for about:config)
MagicSLR (among other things, this gets me a "Send/Receive all" toolbar button)
Fix Tb Titlebar (to get rid of the folder name and just have "Thunderbird" in the title bar; looks better on Windows' task bar)
TB QuickMove (move mails to predefined set of folders easily)
No new window on double click (prevents new window should I accidently dblclick a folder)

My own (see homepage):
Mouse Gestures (As I switched to Tb only recently, I'm not yet used to gestures in mailing very much - opposed to browsing - but it's nice to have)
MessageFaces (Embed an icon-like picture of yours in your mail)

I use these:

http://home.ripway.com/2004-3/80433/stuff2/infolister-thunderbird.html

:D

The only extension I have installed is No New Window on Double Click. Absolutely essential.

None. :-P Thunderbird is just a newsgroup client for me (and email client for checking emails from ISP).

None, for the same reasons as minghong before me.

However, I would use an extension that automatically linkified "bug #", "bug # comment #", and "attachment #" bits of text in plaintext email from *@bugzilla.mozilla.org, tho, if it existed. I don't have the time to figure out how to make such an extension, and I haven't seen one like it, so I don't.

AboutConfig, adding "about:config" item under Tools menu,
Quote Colors, coloring (text and background) different quote levels diff. colours,
Quote Collapse, putting "+" and "-" icons in messages, mimicking Gmail behaviour,
Mail Redirect, allowing redirect, or "bounce" a message to a different recepient,
Minimize to Tray, hiding windows into system tray (Win only I think),
Proxy Button, creating a toolbar button for switching proxy on or off (a must on laptop/notebooks when connecting directly or via LAN)
Tahat's all. TB extensions need more publicity IMO to get out of Firefox's siblings' shade;-).

I could live without most of these extensions, but as I use them sometimes I still keep them. Here is the list by the level of importance:

TB Attachment Tools
Quote Collapse
TB Header Tools
URL Link
Virtual Identity

Oh geeze, I have about a million. :-)

Most important:
Mnenhy (custom header views, ROT13 and other codecs, customizable column layouts per folder/newsgroup, X-face display, registry viewer, chrome manager, clickable reference headers, etc. I could go on. :-) )

Others:
AboutConfig(pretty obvious)
Folderpane Tools (rearrange accounts)
MessageID-Finder(makes clicking on message-IDs work. You can even have it go to google groups)
Quote Colors(display colours based on quote level)
ChromEdit(UI for access to userChrome.css, userContent.css, and user.js file)
Buttons!(adds more buttons available for the toolbars)
MagicSLR(adds toolbar buttons for offline features)
Attachment Tools(remove attachments from a message, etc.)
FoxyTunes(control media players from Thunderbird)
Add Bcc Button(adds a Bcc button in the contacts pane of the compose window)
Launchy(open current page, links, mailto, images and view source with external applications.)
ConfigDate(customize the date display format)
MessageFaces(add and view "Face:" headers)
quotecollapse(collapses quoted text by quote level)
Customizable Toolbar Buttons(create your own toolbar buttons)
Grippies(brings back grippies)

I use Display Mail User Agent Extension, when I feel curious as to which client my correspondent uses but can't be bothered to read the headers =)
http://www.cweiske.de/misc_extensions.htm#dispMUA

Folderpane Tools 0.0.1 is a cool one IMO, will let you reorder the folder pane and the tree in the account setting dialog, as well as set an other account as default.
http://www.chuonthis.com/extensions/folderpane.php

BUT the one I can't live without must be Thunderbird-Tray (start from/minimize to tray)
http://sniperbeamer.de/tbtray/

PS. I concur with Jeff about the "linkifying" extension!

Ooh, some of these extensions are pretty cool. :)

Enigmail the best =:)

Now two. Thanks to Web-Mail, I should no longer need an external program to download Hotmail to Thunderbird.

mozPod 0.1 dev ;-) (awaiting your iconage).

MagicSLR
adressContext

Asa, thanks for the tip. Now if they just add Gmail, I can ditch MrPostman completely. The only other extension I use is Enigmail, but given the impressive listing from others, that might change shortly.

You mean GMail-support by the WebMail-extension ? Thunderbird can support GMail natively when you enable POP/SMTP in you GMail-account.

I have only one extension installed in my t'bird but that one is tremendously important to me since I have to reply to several dozens of users' emails per day : it's Bart Decrem's extension adding a "Send Later" to new messages toolbar. Only one minor nit : he forgot to add the styles for the Customize Toolbar panel and it does not look nice at all there. Otherwise, works like a charm.

I think a webmail extension is a great idea, but how will it handle changing interfaces? Hotmail in particular likes to change the interface frequently to break proxy programs.

None yet, but this well-conceived post has shown me some that could be helpful.

Say, a friend and workmate would like to be be able to save the text of selected emails to one text file. Does anything like that exist? It's an Outlook-parity thing for him.

I use:
Minimize to Tray, Mailredirect and my own Quicktext (http://www.hesslow.se/quicktext) to insert predefined texts and it also support some variables.

SenderFace is an extension that actually attracts the less-technical to Thunderbird because it just makes it prettier and easier to use by showing the face of the sender on the messages.

I also like MinimizeToTray, for convenience.

BTW, are the confusing TB menus being redesigned for 1.1? I have a few suggestion on my SFX blog.

The webmail homepage states that it's for "Tb v1 only." Does this mean that it won't work on any of the higher versions or do they mean 1.0 and up?

MarbleheadMan: I don't know if TBird's menus are being re-done for 1.1, but you can edit your own. See http://menueditor.mozdev.org/ for MenuEditor, and extension for Firefox and Thunderbird that will allow you to re-arrange menu entries to your liking.
______
Dennis

I use a lot of different extensions but I have just discovered (the excellent) SyncKolab which "reads a selected IMAP folder and synchronizes it with the local address book and calendar".

The calendar extension must be installed first.

Image Zoom
MagicSLR
Minimize to Tray
Quote Colors

:-)

About config and Mozilla calendar

Freeform From and FCC on Compose v0.1.5 (TB 1.0 ok)

This extension saves replies and forwards in the same folder as the original message. This is essential if you like to view your messages as conversation threads.

This really should be built into Thunderbird because without it viewing messages sorted by threads is a pain - you have to sort manually sort your messages into the correct folders.

• Mail Redirect 0.6 Allow to redirect (a.k.a. "bounce") mail messages to other recipients
• Quick Reply Extension 0.5.5 Supplies a quick reply extension for Mozilla Thunderbird.

just moving from mozilla suite to firefox and thunderbird... so far I mostly wish that someone would port the Orbit 3+1 theme over for FF and TB... (correctly/completely)... that'd be my first extension to load.

http://themes.mozdev.org/themes/orbit.html

asa: thanks for the posting this thread - this will provide good information for the next edition of the book :)

When I was testing extensions for the book, I came across a lot of cool ones - some of the ones I use include:

Virtual Identity extension
MagicSLR

Quickreply is absolutely the most essential!
http://quickreply.mozdev.org/

I tested a lot and keep those very useful ones :
- Mozilla Calendar : to organize my days
- TB Attachment Tools : to remove the too large attachments
- Buttons! : to easily change the SMTP server when I move from home to work
- QuoteCollapse : to easily read newsgroups
- Dictionary Search
- Mail Redirect
- World Weather : for fun

Jeff: There's already an Orbit theme for Fx.

Here's the homepage where you can download it:
http://blog.vollmondlicht.com/firefox.php.

They don't have one for Thunderbird though. Sorry. :\

- Quote Colors
- TB In-line Spell Checker (buggy, but I'm a terrible speller and need autocorrect)
- Open long URL (for those URLs that wrap in email)
- View Headers Toggle (gotta see full headers sometimes)
- Duplicate Message Remover
- MboxImport
- something that a coworker wrote to access the company ldap server and display sender's picture, phone #, and title