This is a list of add-ons I use daily:
Firebug for Web Development
Nagios Checker for server monitoring
The statusbar indicator of the events from the network monitoring system Nagios. What else can be said?
GMail Notifier for email alerts
Since I use Gmail as mail solution for my day-to-day work this is a great add-on. It supports multiple accounts.
Delicious Bookmarks for keeping your bookmarks in one place
Since I use multiple desktops, laptops and other devices I hate to have separate bookmarks for each of them. delicious is a great way to keep them in one place. You can keep private bookmarks if you want.