Firstly if you want web traffic you need the search engines to index your site. And the search engines look for keywords. So it is really useful to know the keywords that people are using in their search queries. Google release a lot of this information in Google Zeitgeist. Using this could make your website a lot more successful.
Secondly there is a lot of useful information you can find out about a website but it takes ages to do so. Firefox SEO does it automatically to every item on a Google search. This really is a very powerful tool indeed and will give you so much marketing information that you probably didn’t have before. Here is what it looks for: PR: ? | Google Cache Date: ? | Traffic Value: ? | Age: ? | del.icio.us: ? | del.icio.us Page Bookmarks: ? | Diggs: ? | Digg’s Popular Stories: ? | Stumbleupon: ? | Twitter: ? | | | | | | Technorati: ? | Alexa: ? | Compete.com Rank: ? | Compete.com Uniques: ? | Trends | Cached: ? | dmoz: ? | Bloglines: ? | | dir.yahoo.com: ? | Botw: ? | Whois | Sktool | Yahoo position: ? | Majestic SEO linkdomain: ?