Moz

If there was to be a winner in this category, Moz is the one SEO tool that you really need. It provides access to social mentions, links to your website, website rankings, and an analysis of website traffic, content, and engagement within one useful tool. Moz is great for uncovering SEO opportunities that you are missing out on, and avoiding critical SEO faux-pas that can damage your search engine presence and drive traffic away from your business. If youÕre looking for access to link data, research tools, and custom reporting, Moz is the all-in-one tool you need.

http://www.moz.com