Following on from my list of firefox extensions, here’s my list of Thunderbird extensions:
Changes the minimise in Thunderbird to send it to the system tray and still check your e-mail in the background.
Adds a list of your contacts below the folder pane
Show Face headers in e-mails and news groups
Makes quotes in e-mail and news messages collapsable to save space
Makes quotes in e-mail and news messages appear in different colours (I used to do this with userContent.css, but this is easier)
Adds extra buttons to Thunderbird including change SMTP server which I use a lot
Adds an extra button to Thunderbird to toggle the viewing of headers
Restores about:config to Thunderbird so you can access the advanced configuration
Lets you bounce e-mail with the original headers
Also you can see which search plugins I use.