Google Webmaster Tools used to identify issues with your site and can even let you know if it has been infected with malware (not something you ever want to see, but if you haven’t spotted it yourself, or had one of your users tweet at you to let you know, it’s invaluable).

Google Webmaster Tools is a really valuable instrument for SEO. It gives you at a glance data about all important SEO aspects of a site, such as keywords, links, crawl errors, etc.