Google Tools

http://www.google.com/webmasters/tools/home?hl=en – Great general SEO tool

http://www.google.com/webmasters/tools/mobile-friendly/ – Tells you if your site is mobile friendly – i.e. responsive

http://www.google.com/analytics/ – Another great general SEO tool

http://adwords.google.com/KeywordPlanner – Tells you what keywords are used the most

http://developers.google.com/speed/pagespeed – Will offer suggestions to speed up rendering of your site (html and css suggestions)