i agree with what most people post.
i personally prefer Chrome to safari or Firefox.
In my experience firefox eventually slows your machine to a grinding halt - isn't that what we switched to OSX from Windows? No thanks
I'm willing to give Safari another try once 10.9 comes out - actually really excited about some of the features they underscored at the keynote!
I really like adblock, mightytext and any.do
in addition i love History Eraser, Command-click fix (essential if you use chrome and shop on Amazon - or any other site that breaks "command-click to open in a new window"... rotten tomatoes is another one, but there are many)
if you're a student, definitely check out evernote and workflowy if you haven't already. I am a law student and i love both.
i'll post more if i think of them