Google's Webmaster Tools help you master the Google Search and put your website at the top of the results. Google is the most popular search engine and you need your website to rank high to receive traffic. Use Webmaster Tools to find any missing data that is hurting your ranking and add in extra information that Google needs to index your site.
SEOMoz is the most popular provider of SEO software for websites. Their tools are simple and allows for a quick and comprehensive guide to the state of your website's SEO. Using best practices from years of SEO mastery, the knowledge of SEOMoz is now available through the use of their online offerings.
Google Analytics are a free tool that helps you measure your advertising ROI as well as track all web assets for your site such as video, social networking, page views, and application stats. Google Analytics tracks in real time visits to your site and lets you understand more about the people who visit your site such as types of device used to access your site, time spent viewing pages on your site, and where your visitor came from geographically.
Marketing Grader (formerly Website Grader) is a free tool to measure and analyze your online marketing efforts. Marketing Grader provides a report on the current state of your website along with actionable steps you can take to make a bigger impact on your visitors. This is a great tool to start with and get the big picture of the effectiveness of your marketing efforts.