If you use the popular Firefox browser then you can download and install some pretty cool SEO extensions for the browser which include the following.

SEO for Firefox extension

The extension is developed and released by the SEO Book web site and offers the following functionality plus more.

PR: (Google PageRank) an estimated measure of global link authority
Age: age pulled from Archive.org, shows the first time a page was indexed by Archive.org's spider. The theory is that if Archive.org found a page so did many of the major search engines.
Links: (Yahoo! linkdomain) shows a rough estimate of the total number of links pointing at a domain
edu Link: (Yahoo! .edu linkdomain ) shows a rough estimate of the total number of .edu links pointing at a domain
edu Page Link: (Yahoo! .edu link ) shows a rough estimate of the total number of .edu links pointing at a specific page
gov Link: (Yahoo! .gov linkdomain ) shows a rough estimate of the total number of .gov links pointing at a domain
Page Links: (Yahoo! link) shows a rough estimate of the total number of links pointing at a page
del.icio.us: number of times a URL has been bookmarked on Del.icio.us. Heavily skewed toward techy / Web 2.0 stuff.
Technorati: an estimate of the total number of links to a site from blogs
Alexa: rank based on website traffic . Heavily skewed toward internet marketing and webmaster related resources.
Cached: (Google site:) shows how many pages from a site are indexed in Google

You can download the SEO for Firefox extension from the SEO Book web site.

SEOpen Extension

SEopen is another extension for Firefox which you may find useful from an SEO prespective.

The extension offers the following features

Yahoo Backlinks
Pages in Yahoo Index
Google Backlinks
Google Cache
Pages in Google index
Google Translate to English
Google Related
PageRank Check
MSN Backlinks
Pages in MSN Index
Alexa Overview
Alexa Traffic
Alexa Related
Alexa Backlinks
"Mass Check" multiple sources at once
Check DMOZ Inclusion
Keyword Density
Page Size Checker
HTML Validator
Server Header Viewer
Wayback Machine
Check robots.txt
Whois Info

To find out more and download the firefox extension visit the SE0pen web site