The Calculator is a good add-on for obvious reasons. If you are doing homework, or just simply need to figure out an equation, get the calculator add-on.
BrowserNation Football is another cool add-on because it gives you quick access to your favorite football team.
Converter seems to be the most interesting add-on because it lets you take a metric unit of measurement, and change it into a customary unit of measurement.
ESPN Cricinfo is a sidebar add-on that is very handy to those sports fans that spend a lot of time working on the computer. When you are working, you can get the latest sports news.
Find Chord is an awesome add-on for those rockin' guitar guys. Like me!! It helps you find a guitar chord if you don't know what it is, or you are having a hard time playing a certain chord.
Free Games is just a simple add-on that is a stress release. A long day can be a lot easier if you spend five short minutes on a computer game.
Gmail Manager is a helpful add-on that lets you manage multiple accounts in Gmail if you happen to have several.
The Mouse Clicker Counter is a fun little add-on that keeps track of your right and left clicks. It is very interesting to see the number of clicks you spend on just one website.
I have posted about personas before, that is an add-on by itself. A persona lets you change the background of your Firefox.
Search image by Google is an add-on that lets you search for something by using the add-on and not having to go to the Google website.