Webmaster Eyes overlays a site with small bars that indicate PageRank, in a similar way with Google Analytics' site overlay. If you click on a green bar, you'll navigate to the link next to the PageRank bar.
It's useful to use Webmaster Eyes with search results, sitemaps (to see PageRank distribution on internal pages) or not-very-familiar sites to see popular articles or blog posts.
To make it easier to access the site, bookmark this link (it's a simple bookmarklet that works in Explorer, Firefox, Opera).
Another way to use Webmaster Eyes is to install Google Toolbar 4 (Internet Explorer only). After that, go to webmastereyes.com, right-click on the search bar and select "Create Custom Search" from the menu. From now on, enter the URLs in the Google search box instead of the address bar and click the button you've just created. If you already visit a site, copy the address from the address bar to Google Toolbar search box and click Webmaster Eyes button. Note that the URL must start with [http://].
Related:
Check link popularity
Google sandbox and TrustRank algorithm
The future of search