I use Seoquake.It shows many seo parameters such as: page info, google PR, google Index, bing index, alexa rank, webarchive age, whois, semrush rank ,semrush price, internal and external links etc
I use just two : one from the list - of course, SEOQuake to shake and wake the rusty dust, and the other tool is the one that beats everything else - my mind.
Great thread and great add-ons shared in here. Some of the add-ons which I'm using are SEOquake, Firebug, Search Status, Autofill form, Inform.