For home, I LOVE adblock plus, site launcher, greasemonkey and greasefire, just to name a few.
For work (I am in IT) I absolutely love my Nagios Checker. It parses data from my Nagios network monitoring software and puts it into a small space in my statusbar. Basically it's green for no problem, yellow for a non critial problem, and red for critical. I can filter out things I don't need like machines that are turned off every night (otherwise shows up critical) and change how info is displayed. I click it and it takes me straight to the alert page on my Nagios web interface. Very nice for alerting me to network and system issues before I even get a call about it. Most of the other add ons are for little things, nothing major, like when viewing the page source for a website, it opens in a new tab rather then a new window.