Add-ons are plugins that create functionality for SquirrelMail which isn't directly email related, for example a calendar or other PDA functions.
Plugins that extend the functionality of the Addressbook.
Administrator's Relief (17)
These plugins do not change anything to how SquirrelMail acts to the user, but do change how squirrelmail behaves to the administrator.
Change password (7)
Allows the user to change his password from the comfort of the SquirrelMail interface, for different kind of authentication mechanisms.
These plugins assist you while you are writing (composing) new emails.
Filters & Spam (15)
These plugins give you filters to sort your incoming mail and the means to battle spam from within SquirrelMail.
Logging in (21)
All kinds of ways to change or extend the login procedure.
Message & Folder Manipulation (17)
Offer more ways to move around, change properties of and navigate messages and folders.
Message View (11)
These plugins extend the viewing of the (received) messages, and the viewing of the attachments.
Plugins that don't fit into other categories.
These plugins have functionality that over time has been integrated into the standard distribution. They're here for archeological purposes; if you've got a somewhat recent SquirrelMail version, you don't need these.
Let the user make auto-reply messages as if the user were on vacation (and indeed may even be!)
Visual Additions (18)
Add (mainly non-interactive) information and graphics to the SquirrelMail interface (excluding weather information, which has its own category)
If you want to see what the weather outside is doing while you're inside using SquirrelMail, you should check one of these plugins.