Free SEO Tools

http://backlinko.com/seo-tools

http://tools.seobook.com/

http://tools.seochat.com/

https://blog.kissmetrics.com/6-indispensable-seo-tools/

http://www.lunametrics.com/blog/2015/06/08/my-15-favorite-free-seo-tools/

http://smallseotools.com/

http://whois.domaintools.com/

http://www.seocentro.com/

http://intodns.com/

http://builtwith.com/

http://iwebchk.com/

http://nibbler.silktide.com/

http://www.seoptimer.com/

https://raventools.com/

https://www.semrush.com/

http://www.spyfu.com/

https://moz.com/tools

https://www.webceo.com/online-seo-tools.htm

https://www.rankwatch.com/

https://www.woorank.com/

https://www.microsoft.com/web/seo/

http://www.webconfs.com/

A few backlink checking tools +

http://www.seoreviewtools.com/website-authority-checker/

http://www.openlinkprofiler.org/

https://www.semrush.com

https://moz.com/researchtools/ose/

http://backlinkwatch.com/

http://buzzsumo.com/ (Content research Tool)