What it was
An on page SEO popup with page metrics, social counts, backlinks and server information. SEOquake and the Ahrefs and Moz toolbars are its modern successors.
Succeeded by SEOquake: On-Page SEO Checker
The earlier extension is gone, but its role is filled today by SEOquake: On-Page SEO Checker, with 1M users and a 3.5 rating. See its statistics and history.