Here's a Firefox extension that does something really useful: it gives information about each result in a Google search or a Yahoo search. SEO for Firefox lists for each result: site's PageRank, the number of del.icio.us bookmarks, the number of backlinks, Alexa rank, domain age and other interesting data. The extension can be customized to show only some of the values. Although the information is valuable, if not for individual values, at least as a whole - the extension can slow down the page load.
