• Get instant access to any information about the page you have opened: Google index, Yahoo! backlinks, Alexa Rank, archive age, Del.icio.us mentions, subpages, source, Whois, keyword density and many more.
• Integrate all the above data in your Google, Yahoo and MSN search results page. Sort the results by any of the parameters.

• Google PageRank, Alexa Rank and Compete.com Rank toolbars for every page you visit.
• Highlight nofollow links, access any page matatags with one click, view the link report and robots.txt info.

• Search Google and see your search history and Google suggestions;
• Access your mail with one click.

SEOMoz Rank Checker button - allows you to check rankings of any page by one click (the tool was also described here).

User Agent Switcher - check your website with the user agent being Googlebot, for example.

• an exhaustive SEO report on each page integrated within your search results page: Google PR, Google cache date, age, Del.icio.us mentions, Y! links, Y! .edu links, Y! .gov links, Y! page links, Compete.com Rank, Compete.com uniques, number of cached pages, Dmoz/Yahoo directory/Botw presence, Whois;
• an instant access to other great SEO research tools: keyword research, GoogleAdwords tool, traffic estimator, Google trends;
• export to CSV function.

Compete.com toolbar - the one I don’t use too often but I like the site profile feature.

Web Developer plugin has plenty of great features; I will mention only those I use most often.

Most useful features:

• shows image Alt text;
• Java disable option;
• shows image and link titles.

Page Age shows age for the page you are located at.