Extensions

Aphrodite
An alternative user interface for Mozilla.
http://aphrodite.mozdev.org/

ChatZilla
An IRC chat client built using the Mozilla framework.
http://chatzilla.hacksrus.com/

Checky 2.2
The Checky plug-in for Mozilla, Firefox and Netscape allows for web content and resource developers to easily access free and commercial online validator services.
http://checky.sourceforge.net/

Extension Room
Available extensions for Mozilla. Also available are Firefox and Thunderbird extensions.
http://extensionroom.mozdev.org/

Googlebar L10N
Localized Google toolbar for Mozilla users.
http://googlebarl10n.mozdev.org/

How to hack extensions to make them compatible with SeaMonkey
Provides a template install script and describe possibles problems when porting extensions.
http://users.skynet.be/fa258499/hackexttutorial.html

IE Ctrl+Enter for Mozilla
Add Internet Explorer-like behaviour for the Ctrl+Enter combination.
http://www.geocities.com/oliversl/mozilla/

Jeremy's Mozilla Extensions
Enhancement extensions created by Jeremy Gillick.
http://mozmonkey.com/

Modified Firefox Extensions
General Firefox extensions that have been modified to work on SeaMonkey.
http://xsidebar.mozdev.org/modifiedmisc.html

mozCalc
Localized desktop calculator application allowing input in both "standard" and reverse polish notation.
http://mozcalc.mozdev.org/

NewsMonster
A news, weblog, and RSS aggregator that runs directly in your Mozilla web browser.
http://www.newsmonster.org/

One Click Dynamic Sidebar
A dynamic sidebar for Mozilla apps that include many programs in one sidebar.
http://oneclick.mozdev.org/

Outliner
Allows organizion of notes into a tree.
http://outliner.mozdev.org

RankQuest SEO Toolbar
Provides quick access to more than 25 SEO tools.
http://www.rankquest.com/download-toolbar.html

ReloadEvery
Automatically reloads a web page every so many seconds or minutes.
http://reloadevery.mozdev.org

SeaMonkey Extensions
Directory of available extensions with user reviews.
https://addons.mozilla.org/en-US/seamonkey

xSidebar
Brings the look and feel of Firefox style sidebars to Seamonkey. In addition, xSidebar also implements the infrastructure (the XUL elements and javascript APIs from Firefox) to make porting Firefox sidebar extensions to Seamonkey as painless as possible.
http://xsidebar.mozdev.org/